Mitigating false messages and effects thereof in multi-chamber leadless pacemaker systems and other imd systems

ABSTRACT

Implantable medical devices (IMDs) described herein, and methods for use therewith described herein, reduce how often an IMD accepts a false message and/or reduce adverse effects of an IMD accepting a false message. Such IMDs can be leadless pacemakers (LPs), or implantable cardio defibrillators (ICDs), but are not limited thereto. Such embodiments can be used help multiple IMDs (e.g., multiple LPs) implanted within a same patient maintain synchronous operation, such as synchronous multi-chamber pacing.

PRIORITY CLAIM

This application is a divisional of U.S. patent application Ser. No.17/022,994, filed Sep. 16, 2020, which claimed priority to U.S.Provisional Patent Application No. 62/907,396, filed Sep. 27, 2019.Priority is claimed to each of the above applications, and each of theabove applications is incorporated herein by reference in its entirety.

RELATED APPLICATION

This application is related to commonly assigned U.S. patent applicationSer. No. 17/022,774, titled MITIGATING FALSE MESSAGES AND EFFECTSTHEREOF IN MULTI-CHAMBER LEADLESS PACEMAKER SYSTEMS AND OTHER IMDSYSTEMS, which issued as U.S. Pat. No. 11,633,610, and which isincorporated herein by reference in its entirety.

FIELD OF TECHNOLOGY

Embodiments described herein generally relate to methods and systems forproviding and improving communication between implantable medicaldevices, one or more of which may be a leadless cardiac pacemaker.

BACKGROUND

Implantable medical devices and systems often rely on propercommunication to operate correctly. For example, in a dual chamberleadless pacemaker system, implant-to-implant (i2i) communication iscritical for proper synchronization and operation of the system.However, noise may cause one or more devices of such a system to falselydetect an i2i message and inappropriately respond thereto. For a morespecific example, noise may cause a ventricular leadless pacemaker (LP)to falsely detect a message from an atrial LP, which then could triggerthe ventricular LP to pace at an inappropriate high-rate, and moregenerally, at inappropriate times. For another example, an atrial LP canfalsely detect a message that causes the atrial LP to pace the rightatrium at a rate that is much higher than a rate at which a ventricularLP is pacing the right ventricle, thereby resulting in unsynchronizedpacing. To reduce the probability of an implantable medical devicefalsely detecting i2i messages, such i2i messages may include redundantdata for error detection and correction. However, due to the desire tokeep the power consumption low, the i2i messaging and/or errorcorrection and detection scheme may be simple and false messages maystill get through.

SUMMARY

Certain embodiments of the present technology are related to methods foruse by a leadless pacemaker (LP) implanted in or on a first cardiacchamber of a patient also having an implantable medical device (IMD)remotely located relative to the LP, wherein the LP is configured topace the first cardiac chamber and adjust a pacing rate at which thefirst cardiac chamber is paced based on a pacing rate indicator includedin an implant-to-implant (i2i) message received from the IMD. Inaccordance with certain embodiments, the method includes the LPmonitoring for i2i messages, and in response to the LP receiving an i2imessage including a pacing rate indicator that would cause an adjustmentto the pacing rate exceeding a rate adjustment threshold, the LPlimiting the adjustment to the pacing rate to a specified amount. Inaccordance with certain embodiments, the specified amount, by which theLP limits an adjustment to the pacing rate in response to receiving ani2i message including a pacing rate indicator that would cause anadjustment to the pacing rate exceeding a rate adjustment threshold,comprises the rate adjustment threshold. In accordance with certainembodiments, the specified amount, by which the LP limits an adjustmentto the pacing rate in response to receiving an i2i message including apacing rate indicator that would cause an adjustment to the pacing rateexceeding a rate adjustment threshold, comprises a predetermined valueor a predetermined function of a present pacing rate. In accordance withcertain embodiments, the LP comprises a first LP (LP1), and the IMDcomprises a second LP (LP2) implanted in or on a second cardiac chamber.For example, the first cardiac chamber comprises a right atrial (RA)chamber and the second cardiac chamber comprises a right ventricular(RV) chamber. In accordance with certain embodiments, the LP isimplanted in or on the RV chamber, and the IMD comprises a subcutaneousimplantable cardioverter defibrillator (S-ICD). In certain suchembodiments, the i2i messages are transmitted and received viaconductive communication.

Certain embodiments of the present technology are related to animplantable LP configured to be implanted in or on a first cardiacchamber of a patient and configured to pace the first cardiac chamberand adjust a pacing rate at which the first cardiac chamber is pacedbased on a pacing rate indicator included in an i2i message receivedfrom an IMD remotely located relative to the LP. Such an LP can includeat least one receiver configured to receive i2i messages, and acontroller configured to limit an adjustment to the pacing rate to aspecified amount, in response to the LP receiving an i2i messageincluding a pacing rate indicator that would cause an adjustment to thepacing rate to exceed a rate adjustment threshold. In accordance withcertain embodiments, the specified amount, by which the controllerlimits an adjustment to the pacing rate in response to receiving an i2imessage including a pacing rate indicator that would cause an adjustmentto the pacing rate exceeding a rate adjustment threshold, comprises therate adjustment threshold. In accordance with certain embodiments, thespecified amount, by which the controller limits an adjustment to thepacing rate in response to receiving an i2i message including a pacingrate indicator that would cause an adjustment to the pacing rateexceeding a rate adjustment threshold, comprises a predetermined valueor a predetermined function of a present pacing rate. In accordance withcertain embodiments, the LP comprises a first LP (LP1), and the IMDcomprises a second LP (LP2) implanted in or on a second cardiac chamber.For example, the first cardiac chamber comprises an RA chamber and thesecond cardiac chamber comprises an RV chamber. In accordance withcertain embodiments, the LP is implanted in or on the RV chamber, andthe IMD comprises an S-ICD. In certain such embodiments, the i2imessages are transmitted and received via conductive communication.

In accordance with certain embodiments where an LP monitors for i2imessages, the LP expects to receive an i2i message within an expectedperiod from an IMD remotely located relative to the LP. In response tothe LP not receiving an i2i message within the expected period, the LPreduces the pacing rate at which the LP paces the first cardiac chamber.In accordance with certain embodiments, the expected period comprises apredetermined period of time, or a predetermined number (N) of cardiaccycles, where N is an integer that is equal to or greater than 1. Inaccordance with certain embodiments, an amount by which the LP reducesthe pacing rate at which the LP paces the first cardiac chamber, inresponse to the LP not receiving an i2i message within the expectedperiod, comprises a predetermined value, or a predetermined function ofa present pacing rate. In certain such embodiments, the LP limits howmuch the pacing rate is reduced, in response to the LP not receiving ani2i message within the expected period, such that the pacing rate willnot fall below a predetermined minimum rate. In accordance with certainembodiments, the LP comprises a first LP (LP1), the IMD comprises asecond LP (LP2) implanted in or on a second cardiac chamber, the firstcardiac chamber comprises an RA chamber, and the second cardiac chambercomprises an RV chamber. In accordance with certain embodiments, the i2imessages are transmitted and received via conductive communication, theLP is implanted in or on the RV chamber, and the IMD comprises an S-ICD.In certain such embodiments, the i2i messages are transmitted andreceived via conductive communication.

In accordance with certain embodiments, an LP comprises at least onereceiver configured to receive i2i messages, and a controller configuredto reduce the pacing rate at which the LP paces the first cardiacchamber, in response to the LP not receiving an i2i message within anexpected period. In accordance with certain embodiments, the expectedperiod comprises a predetermined period of time, or a predeterminednumber (N) of cardiac cycles, where N is an integer that is equal to orgreater than 1. In accordance with certain embodiments, an amount bywhich the controller reduces the pacing rate at which the LP paces thefirst cardiac chamber, in response to the LP not receiving an i2imessage within an expected period, comprises a predetermined value, or apredetermined function of a present pacing rate. In certain suchembodiments, the controller limits how much the pacing rate is reduced,in response to the LP not receiving an i2i message within an expectedperiod, such that the pacing rate will not fall below a predeterminedminimum rate.

Certain embodiments of the present technology are related to a methodinvolving an LP monitoring for i2i messages, and in response to the LPreceiving at least a specified plurality of i2i messages that include asame pacing rate indicator, the LP adjusting the pacing rate at whichthe first cardiac chamber is paced based on the pacing rate indicatorincluded in the specified plurality of i2i messages received by the LP.The method also includes in response to the LP not receiving at leastthe specified plurality of i2i messages that include a same pacing rateindicator, the LP not adjusting the pacing rate at which the firstcardiac chamber is paced based on a pacing rate indicator included in ani2i message received by the LP. In accordance with certain embodiments,the specified plurality of i2i messages comprise at least N consecutivei2i messages that include the same pacing rate indicator, where N is apredetermined integer that is equal to or greater than 2, and the IMD isconfigured to send at least N consecutive i2i messages including a samepacing rate indicator to the LP whenever the IMD changes the rate atwhich the LP paces the first cardiac chamber. In accordance with otherembodiments, the specified plurality of i2i messages comprise at least Mout of N i2i messages that include the same pacing rate indicator, whereM is a predetermined integer that is equal to or greater than 2, andwhere N is a predetermined integer that is greater than M, and the IMDis configured to send at least N i2i messages including a same pacingrate indicator to the LP whenever the IMD changes the rate at which theLP paces the first cardiac chamber. In certain embodiments, the LPcomprises a first LP (LP1), and the IMD comprises a second LP (LP2)implanted in or on a second cardiac chamber. For example, the firstcardiac chamber comprises an RA chamber, and the second cardiac chambercomprises an RV chamber. In other embodiments, the LP is implanted in oron the RV chamber, and the IMD comprises an S-ICD. In certain suchembodiments, the i2i messages are transmitted and received viaconductive communication.

Certain embodiments of the present technology are related to a systemcomprising an LP and an IMD, wherein the LP is configured to beimplanted in or on a first cardiac chamber of a patient and configuredto pace the first cardiac chamber, and the IMD remotely located relativeto the LP. Further, the LP includes a controller configured to adjust apacing rate at which the first cardiac chamber is paced based on pacingrate indicators included in i2i messages received from the IMD. Inresponse to the LP receiving at least a specified plurality of i2imessages that include a same pacing rate indicator, the controller ofthe LP is configured to adjust the pacing rate at which the firstcardiac chamber is paced based on the pacing rate indicator included inthe specified plurality of i2i messages received by the LP. In responseto the LP not receiving at least the specified plurality of i2i messagesthat include a same pacing rate indicator, the controller of the LP isconfigured to not adjust the pacing rate at which the first cardiacchamber is paced based on a pacing rate indicator included in an i2imessage received by the LP. In certain such embodiments, the specifiedplurality of i2i messages comprise at least N consecutive i2i messagesthat include the same pacing rate indicator, where N is a predeterminedinteger that is equal to or greater than 2, and the IMD is configured tosend at least N consecutive i2i messages including a same pacing rateindicator to the LP whenever the IMD changes the rate at which the LP isto pace the first cardiac chamber. In accordance with other embodiments,the specified plurality of i2i messages comprise at least M out of N i2imessages that include the same pacing rate indicator, where M is apredetermined integer that is equal to or greater than 2, and where N isa predetermined integer that is greater than M, and the IMD isconfigured to send at least N i2i messages including a same pacing rateindicator to the LP whenever the IMD changes the rate at which the LPpaces the first cardiac chamber. In certain embodiments, the LPcomprises a first LP (LP1), and the IMD comprises a second LP (LP2)implanted in or on a second cardiac chamber. For example, the firstcardiac chamber comprises an RA chamber, and the second cardiac chambercomprises an RV chamber. In other embodiments, the LP is implanted in oron the RV chamber, and the IMD comprises an S-ICD. In certain suchembodiments, the i2i messages are transmitted and received viaconductive communication.

In a method according to certain embodiments of the present technology,an IMD transmits i2i messages to the LP, wherein a subset of the i2imessages transmitted by the IMD to the LP include pacing rateindicators. In certain such embodiments, the LP monitors for i2imessages and adjusts the pacing rate at which a first cardiac chamber ispaced based on at least some pacing rate indicators included in at leastsome of the i2i messages received by the LP from the IMD. In certainsuch embodiments, i2i messages including pacing rate indicators that aretransmitted from the IMD to the LP include a longer error detection andcorrection code compared to an error detection and correction codeincluded at least some of the i2i messages not including pacing rateindicators that are transmitted from the IMD to the LP. In certain suchembodiments, the error detection and correction codes comprise cyclicredundancy check (CRC) codes. In certain such embodiments, the LPcomprises a first LP (LP1), and the IMD comprises a second LP (LP2)implanted in or on a second cardiac chamber. For example, the firstcardiac chamber comprises an RA chamber, and the second cardiac chambercomprises an RV chamber. In other embodiments the LP is implanted in oron the RV chamber and the IMD comprises an S-ICD. In certain suchembodiments, the i2i messages are transmitted and received viaconductive communication.

In accordance with certain embodiments, a system comprises an LPconfigured to be implanted in or on a first cardiac chamber of a patientand configured to pace the first cardiac chamber, and an IMD remotelylocated relative to the LP. The LP is configured to adjust a pacing rateat which the first cardiac chamber is paced based on pacing rateindicators included in i2i messages received from the IMD. In certainsuch embodiments, the IMD is configured to include a longer errordetection and correction code in i2i messages including pacing rateindicators transmitted by the IMD to the LP, compared to an errordetection and correction code included in at least some of the i2imessages not including pacing rate indicators that are transmitted fromthe IMD to the LP. In certain such embodiments, the error detection andcorrection codes comprise CRC codes. In certain such embodiments, the LPcomprises a first LP (LP1), and the IMD comprises a second LP (LP2)implanted in or on a second cardiac chamber. For example, the firstcardiac chamber comprises an RA chamber, and the second cardiac chambercomprises an RV chamber. In other embodiments the LP is implanted in oron the RV chamber and the IMD comprises an S-ICD. In certain suchembodiments, the i2i messages are transmitted and received viaconductive communication.

This summary is not intended to be a complete description of theembodiments of the present technology. Other features and advantages ofthe embodiments of the present technology will appear from the followingdescription in which the preferred embodiments have been set forth indetail, in conjunction with the accompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present technology relating to both structure andmethod of operation may best be understood by referring to the followingdescription and accompanying drawings, in which similar referencecharacters denote similar elements throughout the several views:

FIG. 1 illustrates a system formed in accordance with certainembodiments herein as implanted in a heart.

FIG. 2 is a block diagram of a single LP in accordance with certainembodiments herein.

FIG. 3 illustrates an LP in accordance with certain embodiments herein.

FIG. 4 is a timing diagram demonstrating one embodiment of implant toimplant (i2i) communication for a paced event.

FIG. 5 is a timing diagram demonstrating one embodiment of i2icommunication for a sensed event.

FIG. 6A is a high level flow diagram that is used to summarize methodsfor providing slew rate protection, according to certain embodiments ofthe present technology.

FIG. 6B is a high level flow diagram that is used to summarize methodsfor providing selective reductions in pacing rate, according to certainembodiments of the present technology.

FIG. 6C is a high level flow diagram that is used to summarize methodsin which multiple i2i messages must be sent to and received by an LPbefore the LP adjusts its pacing rate in response to receiving i2imessages including a pacing rate indicator, according to certainembodiments of the present technology.

FIG. 6D is a high level flow diagram that is used to summarize methodsin which i2i messages that include a pacing rate indicator includelonger error detection and correction codes than certain other types ofmessages, according to certain embodiments of the present technology.

FIG. 7 shows a block diagram of one embodiment of an IMD (e.g., LP or inICD) that is implanted into the patient as part of the implantablecardiac system in accordance with certain embodiments herein.

DETAILED DESCRIPTION

Certain embodiments of the present technology related to implantablemedical devices (IMDs), and methods for use therewith, that reduce howoften false messages are accepted and/or reduce the effects of IMDsreceiving false messages. Such embodiments are especially useful with asystem that includes one or more leadless cardiac pacemakers, but arenot limited to use therewith. Before providing additional details of thespecific embodiments of the present technology mentioned above, anexemplary system in which embodiments of the present technology can beused will first be described with reference to FIGS. 1-5 . Morespecifically, FIGS. 1-5 will be used to describe an exemplary cardiacpacing system, wherein pacing and sensing operations can be performed bymultiple medical devices, which may include one or more leadless cardiacpacemakers, an ICD, such as a subcutaneous-ICD (S-ICD), and/or aprogrammer to reliably and safely coordinate pacing and/or sensingoperations. A leadless cardiac pacemaker can also be referred to moresuccinctly herein as a leadless pacemaker (LP). Where a cardiac pacingsystem includes an S-ICD, the S-ICD may perform certain sensingoperations and may communicate with one or more LPs by sending and/orreceiving messages to and/or from one or more LPs, as can be appreciatedfrom the below discussion. Where a cardiac pacing system includes aprogrammer, the programmer may be used to program one or more IMDs,download information to one or more IMDs, and/or upload information fromone or more IMDs, as can be appreciated from the below description.

FIG. 1 illustrates a system 100 that is configured to be implanted in aheart 101. The system 100 includes two or more leadless pacemakers (LPs)102 a and 102 b located in different chambers of the heart. LP 102 a islocated in a right atrium, while LP 102 b is located in a rightventricle. LPs 102 a and 102 b communicate with one another to informone another of various local physiologic activities, such as localintrinsic events, local paced events and the like. LPs 102 a and 102 bmay be constructed in a similar manner, but operate differently basedupon which chamber LP 102 a or 102 b is located. The LPs 102 a and 102 bmay sometimes be referred to collectively herein as the LPs 102, orindividually as an LP 102.

In certain embodiments, LPs 102 a and 102 b communicate with oneanother, and/or with an ICD 106, by conductive communication through thesame electrodes that are used for sensing and/or delivery of pacingtherapy. The LPs 102 a and 102 b may also be able to use conductivecommunication to communicate with an external device, e.g., a programmer109, having electrodes placed on the skin of a patient within with theLPs 102 a and 102 b are implanted. While not shown (and not preferred,since it would increase the size and power consumption of the LPs 102 aand 102 b), the LPs 102 a and 102 b can potentially include an antennaand/or telemetry coil that would enable them to communicate with oneanother, the ICD 106 and/or an external device using RF or inductivecommunication. While only two LPs are shown in FIG. 1 , it is possiblethat more than two LPs can be implanted in a patient. For example, toprovide for bi-ventricular pacing and/or cardiac resynchronizationtherapy (CRT), in addition to having LPs implanted in the right atrial(RA) chamber and the right ventricular (RV) chamber, a further LP can beimplanted in the left ventricular (LV) chamber.

In some embodiments, one or more LP 102 a can be co-implanted with theICD 106. Each LP 102 a uses two or more electrodes located within, on,or within a few centimeters of the housing of the pacemaker, for pacingand sensing at the cardiac chamber, for bidirectional communication withone another, with the programmer 109, and the ICD 106.

While the methods, devices and systems described herein include examplesprimarily in the context of LPs, it is understood that the methods,devices and systems described herein may be utilized with various othertypes of IMDs. By way of example, the methods, devices and systems maydynamically control communication between various IMDs implanted in ahuman, not just LPs. Certain embodiments enable a first IMD to receivecommunication messages from at least a second IMD through conductivecommunication over at least a first channel. It should also beunderstood that the embodiments described herein can be used forcommunication between more than two IMDs, and are not limited tocommunication between just first and second IMDs. The methods, devicesand systems may also be used for communication between two or more IMDsimplanted within the same chamber that may be the same type of IMD ormay be different types of IMDs. The methods, devices and systems mayalso be used for communication between two or more IMDs in a systemincluding at least one IMD that is not implanted within a cardiacchamber, but rather, is implanted epicardially, transmurally,intravascularly (e.g., coronary sinus), or subcutaneously (e.g., S-ICD),etc.

Referring to FIG. 2 , a block diagram shows an embodiment for portionsof the electronics within LPs 102 a, 102 b configured to provideconductive communication through the sensing/pacing electrode. One ormore of LPs 102 a and 102 b include at least two leadless electrodesconfigured for delivering cardiac pacing pulses, sensing evoked and/ornatural cardiac electrical signals, and uni-directional orbi-directional communication. In FIG. 2 (and FIG. 3 ) the two electrodesshown therein are labeled 108 a and 108 b. Such electrodes can bereferred to collectively as the electrodes 108, or individually as anelectrode 108. An LP 102, or other type of IMD, can include more thantwo electrodes 108, depending upon implementation.

In FIG. 2 , each of the LPs 102 a, 102 b is shown as including first andsecond receivers 120 and 122 that collectively define separate first andsecond communication channels 105 and 107 (FIG. 1 ), (among otherthings) between LPs 102 a and 102 b. Although first and second receivers120 and 122 are depicted, in other embodiments, each LP 102 a, 102 b mayonly include the first receiver 120, or may include additional receiversother than first and second receivers 120 and 122. As will be describedin additional detail below, the pulse generator 116 can function as atransmitter that transmits i2i communication signals using theelectrodes 108. In certain embodiments, LPs 102 a and 102 b maycommunicate over more than just first and second communication channels105 and 107. In certain embodiments, LPs 102 a and 102 b may communicateover one common communication channel 105. More specifically, LPs 102 aand 102 b can communicate conductively over a common physical channelvia the same electrodes 108 that are also used to deliver pacing pulses.Usage of the electrodes 108 for communication enables the one or moreLPs 102 a and 102 b to perform antenna-less and telemetry coil-lesscommunication.

The receivers 120 and 122 can also be referred to, respectively, as alow frequency (LF) receiver 120 and a high frequency (HF) receiver 122,because the receiver 120 is configured to monitor for one or moresignals within a relatively low frequency range (e.g., below 100 kHz)and the receiver 122 is configured to monitor for one or more signalswithin a relatively high frequency range (e.g., above 100 kHz). Incertain embodiments, the receiver 120 (and more specifically, at least aportion thereof) is always enabled and monitoring for a wakeup notice,which can simply be a wakeup pulse, within a specific low frequencyrange (e.g., between 1 kHz and 100 kHz); and the receiver 122 isselectively enabled by the receiver 120. The receiver 120 is configuredto consume less power than the receiver 122 when both the first andsecond receivers are enabled. Accordingly, the receiver 120 can also bereferred to as a low power receiver 120, and the receiver 122 can alsobe referred to as a high power receiver 122. The low power receiver 120is incapable of receiving signals within the relatively high frequencyrange (e.g., above 100 kHz), but consumes significantly less power thanthe high power receiver 122. This way the low power receiver 120 iscapable of always monitoring for a wakeup notice without significantlydepleting the battery (e.g., 114) of the LP. In accordance with certainembodiments, the high power receiver 122 is selectively enabled by thelow power receiver 120, in response to the low power receiver 120receiving a wakeup notice, so that the high power receiver 122 canreceive the higher frequency signals, and thereby handle higher datathroughput needed for effective i2i communication without unnecessarilyand rapidly depleting the battery of the LP (which the high powerreceiver 122 may do if it were always enabled).

In accordance with certain embodiments, when one of the LPs 102 a and102 b senses an intrinsic event or delivers a paced event, thecorresponding LP 102 a, 102 b transmits an implant event message to theother LP 102 a, 102 b. For example, when an atrial LP 102 a senses/pacesan atrial event, the atrial LP 102 a transmits an implant event messageincluding an event marker indicative of a nature of the event (e.g.,intrinsic/sensed atrial event, paced atrial event). When a ventricularLP 102 b senses/paces a ventricular event, the ventricular LP 102 btransmits an implant event message including an event marker indicativeof a nature of the event (e.g., intrinsic/sensed ventricular event,paced ventricular event). In certain embodiments, each LP 102 a, 102 btransmits an implant event message to the other LP 102 a, 102 bpreceding the actual pace pulse so that the remote LP can blank itssense inputs in anticipation of that remote pace pulse (to preventinappropriate crosstalk sensing).

The implant event messages may be formatted in various manners. As oneexample, each event message may include a leading trigger pulse (alsoreferred to as an LP wakeup notice, wakeup pulse or wakeup signal)followed by an event marker. The notice trigger pulse (also referred toas the wakeup notice, wakeup pulse or wakeup signal) is transmitted overa first channel (e.g., with a pulse duration of approximately 10 μs toapproximately 1 ms and/or within a fundamental frequency range ofapproximately 1 kHz to approximately 100 kHz). The notice trigger pulseindicates that an event marker is about to be transmitted over a secondchannel (e.g., within a higher frequency range). The event marker canthen be transmitted over the second channel.

The event markers may include data indicative of one or more events(e.g., a sensed intrinsic atrial activation for an atrial located LP, asensed intrinsic ventricular activation for a ventricular located LP).The event markers may include different markers for intrinsic and pacedevents. The event markers may also indicate start or end times fortimers (e.g., an AV interval, a blanking interval, etc.). Optionally,the implant event message may include a message segment that includesadditional/secondary information.

Optionally, the LP (or other IMD) that receives any i2i communicationsignal from another LP (or other IMD) or from an external device maytransmit a receive acknowledgement indicating that the receiving LP (orother IMD) received the i2i communication signal. In certainembodiments, where an IMD expects to receive an i2i communication signalwithin a window, and fails to receive the i2i communication signalwithin the window, the IMD may transmit a failure-to-receiveacknowledgement indicating that the receiving IMD failed to receive thei2i communication signal. Other variations are also possible and withinthe scope of the embodiments described herein.

The event messages enable the LPs 102 a, 102 b to deliver synchronizedtherapy and additional supportive features (e.g., measurements, etc.).To maintain synchronous therapy, each of the LPs 102 a and 102 b is madeaware (through the event messages) when an event occurs in the chambercontaining the other LP 102 a, 102 b. Some embodiments described hereinprovide efficient and reliable processes to maintain synchronizationbetween LPs 102 a and 102 b without maintaining continuous communicationbetween LPs 102 a and 102 b. In accordance with certain embodimentsherein, low power event messages/signaling may be maintained between LPs102 a and 102 b synchronously or asynchronously.

For synchronous event signaling, LPs 102 a and 102 b may maintainsynchronization and regularly communicate at a specific interval.Synchronous event signaling allows the transmitter and receivers in eachLP 102 a,102 b to use limited (or minimal) power as each LP 102 a, 102 bis only powered for a small fraction of the time in connection withtransmission and reception. For example, LP 102 a, 102 b maytransmit/receive (Tx/Rx) communication messages in time slots havingduration of 10-20 μs, where the Tx/Rx time slots occur periodically(e.g., every 10-20 ms).

LPs 102 a and 102 b may lose synchronization, even in a synchronousevent signaling scheme. As explained herein, features may be included inLPs 102 a and 102 b to maintain device synchronization, and whensynchronization is lost, LPs 102 a and 102 b undergo operations torecover synchronization. Also, synchronous event messages/signaling mayintroduce a delay between transmissions which causes a reaction lag atthe receiving LP 102 a, 102 b. Accordingly, features may be implementedto account for the reaction lag.

During asynchronous event signaling, LPs 102 a and 102 b do not maintaincommunication synchronization. During asynchronous event signaling, oneor more of receivers 120 and 122 of LPs 102 a and 102 b may be “alwayson” (always awake) to search for incoming transmissions. However,maintaining LP receivers 120, 122 in an “always on” (always awake) statepresents challenges as the received signal level often is low due tohigh channel attenuation caused by the patient's anatomy. Further,maintaining the receivers awake will deplete the battery 114 morequickly than may be desirable.

The asynchronous event signaling methods avoid risks associated withlosing synchronization between devices. However, the asynchronous eventsignaling methods utilize additional receiver current betweentransmissions. For purposes of illustration only, a non-limiting exampleis described hereafter. For example, the channel attenuation may beestimated to have a gain of 1/500 to 1/10000. A gain factor may be1/1000th. Transmit current is a design factor in addition to receivercurrent. As an example, the system may allocate one-half of the implantcommunication current budget to the transmitter (e.g., 0.5 μA for eachtransmitter). When LP 102 a, 102 b maintains a transmitter in acontinuous on-state and the electrode load is 500 ohms, a transmittedvoltage may be 2.5V. When an event signal is transmitted at 2.5V, theevent signal is attenuated as it propagates and would appear at LP 102a, 102 b receiver as an amplitude of approximately 0.25 mV.

To overcome the foregoing receive power limit, a pulsed transmissionscheme may be utilized in which communication transmissions occurcorrelated with an event. By way of example, the pulsed transmissionscheme may be simplified such that each transmission constitutes asingle pulse of a select amplitude and width.

In accordance with certain embodiments herein, LPs 102 a and 102 b mayutilize multi-stage receivers that implement a staged receiver wakeupscheme in order to improve reliability yet remain power efficient. Eachof LPs 102 a and 102 b may include first and second receivers 120 and122 that operate with different first and second activation protocolsand different first and second receive channels. For example, firstreceiver 120 may be assigned a first activation protocol that is “alwayson” (also referred to as always awake) and that listens over a firstreceive channel that has a lower fundamental frequency range/pulseduration (e.g., 1 kHz to 100 kHz/10 μs to approximately 1 ms) ascompared to the fundamental frequency range (e.g., greater than 100kHz/less than 10 μs per pulse) assigned to the second receive channel.

In accordance with certain embodiments, the first receiver 120 maymaintain the first channel active (awake) at all times (including whenthe second channel is inactive (asleep)) in order to listen for messagesfrom a remote LP. The second receiver 122 may be assigned a secondactivation protocol that is a triggered protocol, in which the secondreceiver 122 becomes active (awake) in response to detection of triggerevents over the first receive channel (e.g., when the incoming signalcorresponds to the LP wakeup notice, activating the second channel atthe local LP). The terms active, awake and enabled are usedinterchangeably herein.

Still referring to FIG. 2 , each LP 102 a, 102 b is shown as including acontroller 112 and a pulse generator 116. The controller 112 caninclude, e.g., a microprocessor (or equivalent control circuitry), RAMand/or ROM memory, logic and timing circuitry, state machine circuitry,and I/O circuitry, but is not limited thereto. The controller 112 canfurther include, e.g., timing control circuitry to control the timing ofthe stimulation pulses (e.g., pacing rate, atrio-ventricular (AV) delay,atrial interconduction (A-A) delay, or ventricular interconduction (V-V)delay, etc.). Such timing control circuitry may also be used for thetiming of refractory periods, blanking intervals, noise detectionwindows, evoked response windows, alert intervals, marker channeltiming, and so on. The controller 112 can further include otherdedicated circuitry and/or firmware/software components that assist inmonitoring various conditions of the patient's heart and managing pacingtherapies. The controller 112 and the pulse generator 116 may beconfigured to transmit event messages, via the electrodes 108, in amanner that does not inadvertently capture the heart in the chamberwhere LP 102 a, 102 b is located, such as when the associated chamber isnot in a refractory state. In addition, a LP 102 a, 102 b that receivesan event message may enter an “event refractory” state (or eventblanking state) following receipt of the event message. The eventrefractory/blanking state may be set to extend for a determined periodof time after receipt of an event message in order to avoid thereceiving LP 102 a, 102 b from inadvertently sensing another signal asan event message that might otherwise cause retriggering. For example,the receiving LP 102 a, 102 b may detect a measurement pulse fromanother LP 102 a, 102 b or programmer 109.

In accordance with certain embodiments herein, programmer 109 maycommunicate over a programmer-to-LP channel, with LP 102 a, 102 butilizing the same communication scheme. The external programmer maylisten to the event message transmitted between LP 102 a, 102 b andsynchronize programmer to implant communication such that programmer 109does not transmit communication signals 113 until after an implant toimplant messaging sequence is completed.

In accordance with certain embodiments, LP 102 a, 102 b may combinetransmit operations with therapy. The transmit event marker may beconfigured to have similar characteristics in amplitude and pulse-widthto a pacing pulse and LP 102 a, 102 b may use the energy in the eventmessages to help capture the heart. For example, a pacing pulse maynormally be delivered with pacing parameters of 2.5V amplitude, 500 ohmimpedance, 60 bpm pacing rate, 0.4 ms pulse-width. The foregoing pacingparameters correspond to a current draw of about 1.9 μA. The same LP 102a, 102 b may implement an event message utilizing event signalingparameters for amplitude, pulse-width, pulse rate, etc. that correspondto a current draw of approximately 0.5 μA for transmit.

LP 102 a, 102 b may combine the event message transmissions with pacingpulses. For example, LP 102 a, 102 b may use a 50 μs wakeup transmitpulse having an amplitude of 2.5V which would draw 250 nC (nanoCoulombs) for an electrode load of 500 ohm. The pulses of the transmitevent message may be followed by an event message encoded with asequence of short duration pulses (for example 16, 2 μs on/off bits)which would draw an additional 80 nC. The event message pulse would thenbe followed by the remaining pulse-width needed to reach an equivalentcharge of a nominal 0.4 ms pace pulse. In this case, the currentnecessary to transmit the marker is essentially free as it was used toachieve the necessary pace capture anyhow. With this method, the savingsin transmit current could be budgeted for the receiver or would allowfor additional longevity.

When LP 102 a or 102 b senses an intrinsic event, it can send aqualitatively similar event pulse sequence (but indicative of a sensedevent) without adding the pace pulse remainder. As LP 102 a, 102 blongevity calculations are designed based on the assumption that LP 102a, 102 b will deliver pacing therapy 100% of the time, transmitting anintrinsic event marker to another LP 102 a, 102 b will not impact thenominal calculated LP longevity.

In some embodiments, LP 102 a, 102 b may deliver pacing pulses atrelatively low amplitude. When low amplitude pacing pulses are used, thepower budget for event messages may be modified to be a larger portionof the overall device energy budget. As the pacing pulse amplitude islowered closer to amplitude of event messages, LP 102 a, 102 b increasesan extent to which LP 102 a, 102 b uses the event messages as part ofthe pacing therapy (also referred to as sharing “capture charge” and“transmit charge”). As an example, if the nominal pacing voltage can belowered to <1.25 V, then a “supply halving” pacing charge circuit couldreduce the battery current draw by approximately 50%. A 1.25V pace pulsewould save 1.5 μA of pacing current budget. With lower pulse amplitudes,LP 102 a, 102 b may use larger pulse-widths.

By combining event messages and low power pacing, LP 102 a, 102 b mayrealize additional longevity. Today longevity standards provide that thelongevity to be specified based on a therapy that utilizes 2.5Vamplitude, 0.4 ms pulses at 100% pacing. Optionally, a new standard maybe established based on pacing pulses that deliver lower amplitudeand/or shorter pacing pulses.

While not shown, a communication capacitor can be provided in LP 102 a,102 b. The communication capacitor may be used to transmit event signalshaving higher voltage for the event message pulses to improvecommunication, such as when the LPs 102 a and 102 b experiencedifficulty sensing event messages. The high voltage event signaling maybe used for implants with high signal attenuation or in the case of aretry for an ARQ (automatic repeat request) handshaking scheme.

In some embodiments, the individual LP 102 a can comprise a hermetichousing 110 configured for placement on or attachment to the inside oroutside of a cardiac chamber and at least two leadless electrodes 108proximal to the housing 110 and configured for bidirectionalcommunication with at least one other device 106 within or outside thebody.

FIG. 2 depicts a single LP 102 a (or 102 b) and shows the LP'sfunctional elements substantially enclosed in a hermetic housing 110.The LP 102 a (or 102 b) has at least two electrodes 108 located within,on, or near the housing 110, for delivering pacing pulses to and sensingelectrical activity from the muscle of the cardiac chamber, and forbidirectional communication with at least one other device within oroutside the body. Hermetic feedthroughs 130, 131 conduct electrodesignals through the housing 110. The housing 110 contains a primarybattery 114 to supply power for pacing, sensing, and communication. Thehousing 110 also contains circuits 132 for sensing cardiac activity fromthe electrodes 108, receivers 120, 122 for receiving information from atleast one other device via the electrodes 108, and the pulse generator116 for generating pacing pulses for delivery via the electrodes 108 andalso for transmitting information to at least one other device via theelectrodes 108. The housing 110 can further contain circuits formonitoring device health, for example a battery current monitor 136 anda battery voltage monitor 138, and can contain circuits for controllingoperations in a predetermined manner.

The electrodes 108 can be configured to communicate bidirectionallyamong the multiple leadless cardiac pacemakers and/or the implanted ICD106 to coordinate pacing pulse delivery and optionally other therapeuticor diagnostic features using messages that identify an event at anindividual pacemaker originating the message and a pacemaker receivingthe message react as directed by the message depending on the origin ofthe message. An LP 102 a, 102 b that receives the event message reactsas directed by the event message depending on the message origin orlocation. In some embodiments or conditions, the two or more leadlesselectrodes 108 can be configured to communicate bidirectionally amongthe one or more leadless cardiac pacemakers 102 a and/or the ICD 106 andtransmit data including designated codes for events detected or createdby an individual pacemaker. Individual pacemakers can be configured toissue a unique code corresponding to an event type and a location of thesending pacemaker.

In some embodiments, an individual LP 102 a, 102 b can be configured todeliver a pacing pulse with an event message encoded therein, with acode assigned according to pacemaker location and configured to transmita message to one or more other leadless cardiac pacemakers via the eventmessage coded pacing pulse. The pacemaker or pacemakers receiving themessage are adapted to respond to the message in a predetermined mannerdepending on type and location of the event.

Moreover, information communicated on the incoming channel can alsoinclude an event message from another leadless cardiac pacemakersignifying that the other leadless cardiac pacemaker has sensed aheartbeat or has delivered a pacing pulse, and identifies the locationof the other pacemaker. For example, LP 102 b may receive and relay anevent message from LP 102 a to the programmer. Similarly, informationcommunicated on the outgoing channel can also include a message toanother leadless cardiac pacemaker or pacemakers, or to the ICD, thatthe sending leadless cardiac pacemaker has sensed a heartbeat or hasdelivered a pacing pulse at the location of the sending pacemaker.

Referring again to FIGS. 1 and 2 , the cardiac pacing system 100 maycomprise an ICD 106 in addition to one or more LPs 102 a, 102 bconfigured for implantation in electrical contact with a cardiac chamberand for performing cardiac rhythm management functions in combinationwith the implantable ICD 106. The implantable ICD 106 and the one ormore LPs 102 a, 102 b configured for leadless intercommunication byinformation conduction through body tissue and/or wireless transmissionbetween transmitters and receivers in accordance with the discussedherein.

In a further embodiment, a cardiac pacing system 100 comprises at leastone LP 102 a, 102 b configured for implantation in electrical contactwith a cardiac chamber and configured to perform cardiac pacingfunctions in combination with the co-implanted ICD 106. The leadlesscardiac pacemaker or pacemakers 102 a comprise at least two leadlesselectrodes 108 configured for delivering cardiac pacing pulses, sensingevoked and/or natural cardiac electrical signals, and transmittinginformation to the co-implanted ICD 106.

As shown in the illustrative embodiments, a leadless cardiac pacemaker102 a, 102 b can comprise two or more leadless electrodes 108 configuredfor delivering cardiac pacing pulses, sensing evoked and/or naturalcardiac electrical signals, and bidirectionally communicating with theco-implanted ICD 106.

LP 102 a, 102 b can be configured for operation in a particular locationand a particular functionality at manufacture and/or at programming byan external programmer. Bidirectional communication among the multipleleadless cardiac pacemakers can be arranged to communicate notificationof a sensed heartbeat or delivered pacing pulse event and encoding typeand location of the event to another implanted pacemaker or pacemakers.LP 102 a, 102 b receiving the communication decode the information andrespond depending on location of the receiving pacemaker andpredetermined system functionality.

In some embodiments, the LPs 102 a and 102 b are configured to beimplantable in any chamber of the heart, namely either atrium (RA, LA)or either ventricle (RV, LV). Furthermore, for dual-chamberconfigurations, multiple LPs may be co-implanted (e.g., one in the RAand one in the RV, one in the RV and one in the coronary sinus proximatethe LV). Certain pacemaker parameters and functions depend on (orassume) knowledge of the chamber in which the pacemaker is implanted(and thus with which the LP is interacting; e.g., pacing and/orsensing). Some non-limiting examples include sensing sensitivity, anevoked response algorithm, use of AF suppression in a local chamber,blanking & refractory periods, etc. Accordingly, each LP needs to knowan identity of the chamber in which the LP is implanted, and processesmay be implemented to automatically identify a local chamber associatedwith each LP.

Processes for chamber identification may also be applied to subcutaneouspacemakers, ICDs, with leads and the like. A device with one or moreimplanted leads, identification and/or confirmation of the chamber intowhich the lead was implanted could be useful in several pertinentscenarios. For example, for a DR or CRT device, automatic identificationand confirmation could mitigate against the possibility of the clinicianinadvertently placing the V lead into the A port of the implantablemedical device, and vice-versa. As another example, for an SR device,automatic identification of implanted chamber could enable the deviceand/or programmer to select and present the proper subset of pacingmodes (e.g., AAI or VVI), and for the IPG to utilize the proper set ofsettings and algorithms (e.g., V-AutoCapture vs ACap-Confirm, sensingsensitivities, etc.).

Also shown in FIG. 2 , the primary battery 114 has positive terminal 140and negative terminal 142. Current from the positive terminal 140 ofprimary battery 114 flows through a shunt 144 to a regulator circuit 146to create a positive voltage supply 148 suitable for powering theremaining circuitry of the pacemaker 102. The shunt 144 enables thebattery current monitor 136 to provide the processor 112 with anindication of battery current drain and indirectly of device health. Theillustrative power supply can be a primary battery 114.

Referring to FIG. 2 , the LP is shown as including a temperature sensor152. The temperature sensor can be any one of various different types ofwell-known temperature sensors, or can be a future developed temperaturesensor. For one example, the temperature sensor 152 can be a thermistor,a thermocouple, a resistance thermometer, or a silicon bandgaptemperature sensor, but is not limited thereto. Regardless of how thetemperature sensor 152 is implemented, it is preferably that thetemperature sensed by the sensor is provided to the controller 112 as adigital signal indicative of the blood temperature of the patient withinwhich the LP is implanted. The temperature sensor 152 can behermetically sealed within the housing 110, but that need not be thecase. The temperature sensor 152 can be used in various manners. Forexample, the temperature sensor 152 can be used to detect an activitylevel of the patient to adjust a pacing rate, i.e., for use in rateresponsive pacing. When a person starts to exercise their core bodytemperature initially dips, and then after exercising for a prolongedperiod of time the person's core body temperature will eventually rise.Thereafter, when the person stops exercising their core body temperaturewill return to its baseline. Accordingly, the controller 112 can beconfigured to detect an activity level of a patient based on core bloodtemperature measurements obtained using the temperature sensor 152.

Referring to FIG. 2 , the LP is also shown as including an accelerometer154 which can be hermetically contained within the housing 110. Theaccelerometer 154 can be any one of various different types ofwell-known accelerometers, or can be a future developed accelerometer.For one example, the accelerometer 154 can be or include, e.g., a MEMS(micro-electromechanical system) multi-axis accelerometer of the typeexploiting capacitive or optical cantilever beam techniques, or apiezoelectric accelerometer that employs the piezoelectric effect ofcertain materials to measure dynamic changes in mechanical variables.For example, the accelerometer 154 can be used to detect an activitylevel of the patient to adjust a pacing rate, i.e., for use in rateresponsive pacing. It would also be possible to use outputs of both theaccelerometer 154 and the temperature sensor 152 to monitor the activitylevel of a patient. Alternatively, or additionally, a patient's activitylevel can be monitored based on their heart rate, as detected from anIEGM sensed using the electrodes 108, and/or sensed using aplethysmography signal obtained using a plethysmography sensor (notshown) or a heart sound sensor (not shown), but not limited thereto. Oneor more signals produced and output by the accelerometer 154 may beanalyzed with respect to frequency content, energy, duration, amplitudeand/or other characteristics. Such signals may or may not be amplifiedand/or filtered prior to being analyzed. For example, filtering may beperformed using lowpass, highpass and/or bandpass filters. The signalsoutput by the accelerometer 154 can be analog signals, which can beanalyzed in the analog domain, or can be converted to digital signals(by an analog-to-digital converter) and analyzed in the digital domain.Alternatively, the signals output by the accelerometer 154 can alreadybe in the digital domain. The one or more signals output by theaccelerometer 154 can be analyzed by the controller 112 and/or othercircuitry. In certain embodiments, the accelerometer 154 is packagedalong with an integrated circuit (IC) that is designed to analyze thesignal(s) it generates. In such embodiments, one or more outputs of thepackaged sensor/IC can be an indication of acceleration along one ormore axes. In other embodiments, the accelerometer 154 can be packagedalong with an IC that performs signal conditioning (e.g., amplificationand/or filtering), performs analog-to-digital conversions, and storesdigital data (indicative of the sensor output) in memory (e.g., RAM,which may or may not be within the same package). In such embodiments,the controller 112 or other circuitry can read the digital data from thememory and analyze the digital data. Other variations are also possible,and within the scope of embodiments of the present technology. Inaccordance with certain embodiments of the present technology, describedin additional detail below, a sensor signal produced by theaccelerometer 154 of an LP implanted in or on a cardiac chamber can beused to detect mechanical cardiac activity associated with anothercardiac chamber.

In various embodiments, LP 102 a, 102 b can manage power consumption todraw limited power from the battery, thereby reducing device volume.Each circuit in the system can be designed to avoid large peak currents.For example, cardiac pacing can be achieved by discharging a tankcapacitor (not shown) across the pacing electrodes. Recharging of thetank capacitor is typically controlled by a charge pump circuit. In aparticular embodiment, the charge pump circuit is throttled to rechargethe tank capacitor at constant power from the battery.

In some embodiments, the controller 112 in one leadless cardiacpacemaker 102 a can access signals on the electrodes 108 and can examineoutput pulse duration from another pacemaker for usage as a signaturefor determining triggering information validity and, for a signaturearriving within predetermined limits, activating delivery of a pacingpulse following a predetermined delay of zero or more milliseconds. Thepredetermined delay can be preset at manufacture, programmed via anexternal programmer, or determined by adaptive monitoring to facilitaterecognition of the triggering signal and discriminating the triggeringsignal from noise. In some embodiments or in some conditions, thecontroller 112 can examine output pulse waveform from another leadlesscardiac pacemaker for usage as a signature for determining triggeringinformation validity and, for a signature arriving within predeterminedlimits, activating delivery of a pacing pulse following a predetermineddelay of zero or more milliseconds.

FIG. 2 shows an LP 102 a, 102 b. The LP can include a hermetic housing202 with electrodes 108 a and 108 b disposed thereon. As shown,electrode 108 a can be separated from but surrounded partially by afixation mechanism 205, and the electrode 108 b can be disposed on thehousing 202. The fixation mechanism 205 can be a fixation helix, aplurality of hooks, barbs, or other attaching features configured toattach the pacemaker to tissue, such as heart tissue. The electrodes 108a and 108 b are examples of the electrodes 108 shown in and discussedabove with reference to FIG. 2 .

The housing can also include an electronics compartment 210 within thehousing that contains the electronic components necessary for operationof the pacemaker, including, e.g., a pulse generator, receiver, abattery, and a processor for operation. The hermetic housing 202 can beadapted to be implanted on or in a human heart, and can be cylindricallyshaped, rectangular, spherical, or any other appropriate shapes, forexample.

The housing can comprise a conductive, biocompatible, inert, andanodically safe material such as titanium, 316L stainless steel, orother similar materials. The housing can further comprise an insulatordisposed on the conductive material to separate electrodes 108 a and 108b. The insulator can be an insulative coating on a portion of thehousing between the electrodes, and can comprise materials such assilicone, polyurethane, parylene, or another biocompatible electricalinsulator commonly used for implantable medical devices. In theembodiment of FIG. 2 , a single insulator 208 is disposed along theportion of the housing between electrodes 108 a and 108 b. In someembodiments, the housing itself can comprise an insulator instead of aconductor, such as an alumina ceramic or other similar materials, andthe electrodes can be disposed upon the housing.

As shown in FIG. 2 , the pacemaker can further include a header assembly212 to isolate 108 a and 108 b. The header assembly 212 can be made fromPEEK, tecothane or another biocompatible plastic, and can contain aceramic to metal feedthrough, a glass to metal feedthrough, or otherappropriate feedthrough insulator as known in the art.

The electrodes 108 a and 108 b can comprise pace/sense electrodes, orreturn electrodes. A low-polarization coating can be applied to theelectrodes, such as sintered platinum, platinum-iridium, iridium,iridium-oxide, titanium-nitride, carbon, or other materials commonlyused to reduce polarization effects, for example. In FIG. 2 , electrode108 a can be a pace/sense electrode and electrode 108 b can be a returnelectrode. The electrode 108 b can be a portion of the conductivehousing 202 that does not include an insulator 208.

Several techniques and structures can be used for attaching the housing202 to the interior or exterior wall of the heart. A helical fixationmechanism 205, can enable insertion of the device endocardially orepicardially through a guiding catheter. A torqueable catheter can beused to rotate the housing and force the fixation device into hearttissue, thus affixing the fixation device (and also the electrode 108 ain FIG. 2 ) into contact with stimulable tissue. Electrode 108 b canserve as an indifferent electrode for sensing and pacing. The fixationmechanism may be coated partially or in full for electrical insulation,and a steroid-eluting matrix may be included on or near the device tominimize fibrotic reaction, as is known in conventional pacingelectrode-leads.

Implant-to-Implant Event Messaging

LPs 102 a and 102 b can utilize implant-to-implant (i2i) communicationthrough event messages to coordinate operation with one another invarious manners. The terms i2i communication, i2i event messages, andi2i event markers are used interchangeably herein to refer to eventrelated messages and IMD/IMD operation related messages transmitted froman implanted device and directed to another implanted device (althoughexternal devices, e.g., a programmer, may also receive i2i eventmessages). In certain embodiments, LP 102 a and LP 102 b operate as twoindependent leadless pacers maintaining beat-to-beat dual-chamberfunctionality via a “Master/Slave” operational configuration. Fordescriptive purposes, the ventricular LP 102 b shall be referred to as“vLP” and the atrial LP 102 a shall be referred to as “aLP”. The LP 102that is designated as the master device (e.g. vLP) may implement all ormost dual-chamber diagnostic and therapy determination algorithms. Forpurposes of the following illustration, it is assumed that the vLP is a“master” device, while the aLP is a “slave” device. Alternatively, theaLP may be designated as the master device, while the vLP may bedesignated as the slave device. The master device orchestrates most orall decision-making and timing determinations (including, for example,rate-response changes).

In accordance with certain embodiments, methods are provided forcoordinating operation between first and second leadless pacemakers(LPs) configured to be implanted entirely within first and secondchambers of the heart. A method transmits an event marker throughconductive communication through electrodes located along a housing ofthe first LP, the event marker indicative of one of a local paced orsensed event. The method detects, over a sensing channel, the eventmarker at the second LP. The method identifies the event marker at thesecond LP based on a predetermined pattern configured to indicate thatan event of interest has occurred in a remote chamber. In response tothe identifying operation, the method initiates a related action in thesecond LP.

FIG. 4 is a timing diagram 400 demonstrating one example of an i2icommunication for a paced event. The i2i communication may betransmitted, for example, from LP 102 a to LP 102 b. As shown in FIG. 4, in this embodiment, an i2i transmission 402 is sent prior to deliveryof a pace pulse 404 by the transmitting LP (e.g., LP 102). This enablesthe receiving LP (e.g., LP 102 b) to prepare for the remote delivery ofthe pace pulse. The i2i transmission 402 includes an envelope 406 thatmay include one or more individual pulses. For example, in thisembodiment, envelope 406 includes a low frequency pulse 408 followed bya high frequency pulse train 410. Low frequency pulse 408 lasts for aperiod T_(i2iLF), and high frequency pulse train 410 lasts for a periodT_(i2iHF). The end of low frequency pulse 408 and the beginning of highfrequency pulse train 410 are separated by a gap period, T_(i2iGap).

As shown in FIG. 4 , the i2i transmission 402 lasts for a period Ti2iP,and pace pulse 404 lasts for a period Tpace. The end of i2i transmission402 and the beginning of pace pulse 404 are separated by a delay period,TdelayP. The delay period may be, for example, between approximately 0.0and 10.0 milliseconds (ms), particularly between approximately 0.1 msand 2.0 ms, and more particularly approximately 1.0 ms. The termapproximately, as used herein, means+/−10% of a specified value.

FIG. 5 is a timing diagram 500 demonstrating one example of an i2icommunication for a sensed event. The i2i communication may betransmitted, for example, from LP 102 a to LP 102 b. As shown in FIG. 5, in this embodiment, the transmitting LP (e.g., LP 102 a detects thesensed event when a sensed intrinsic activation 502 crosses a sensethreshold 504. A predetermined delay period, T_(delayS), after thedetection, the transmitting LP transmits an i2i transmission 506 thatlasts a predetermined period T_(i2iS). The delay period may be, forexample, between approximately 0.0 and 10.0 milliseconds (ms),particularly between approximately 0.1 ms and 2.0 ms, and moreparticularly approximately 1.0 ms.

As with i2i transmission 402, i2i transmission 506 may include anenvelope that may include one or more individual pulses. For example,similar to envelope 406, the envelope of i2i transmission 506 mayinclude a low frequency pulse followed by a high frequency pulse train.

Optionally, wherein the first LP is located in an atrium and the secondLP is located in a ventricle, the first LP produces an AS/AP eventmarker to indicate that an atrial sensed (AS) event or atrial paced (AP)event has occurred or will occur in the immediate future. For example,the AS and AP event markers may be transmitted following thecorresponding AS or AP event. Alternatively, the first LP may transmitthe AP event marker slightly prior to delivering an atrial pacing pulse.Alternatively, wherein the first LP is located in an atrium and thesecond LP is located in a ventricle, the second LP initiates anatrioventricular (AV) interval after receiving an AS or AP event markerfrom the first LP; and initiates a post atrial ventricular blanking(PAVB) interval after receiving an AP event marker from the first LP.

Optionally, the first and second LPs may operate in a “pure”master/slave relation, where the master LP delivers “command” markers inaddition to or in place of “event” markers. A command marker directs theslave LP to perform an action such as to deliver a pacing pulse and thelike. For example, when a slave LP is located in an atrium and a masterLP is located in a ventricle, in a pure master/slave relation, the slaveLP delivers an immediate pacing pulse to the atrium when receiving an APcommand marker from the master LP.

In accordance with some embodiments, communication and synchronizationbetween the aLP and vLP is implemented via conducted communication ofmarkers/commands in the event messages (per i2i communication protocol).As explained above, conducted communication represents event messagestransmitted from the sensing/pacing electrodes at frequencies outsidethe RF or Wi-Fi frequency range. Alternatively, the event messages maybe conveyed over communication channels operating in the RF or Wi-Fifrequency range. The figures and corresponding description belowillustrate non-limiting examples of markers that may be transmitted inevent messages. The figures and corresponding description below alsoinclude the description of the markers and examples of results thatoccur in the LP that receives the event message. Table 1 representsexemplary event markers sent from the aLP to the vLP, while Table 2represents exemplary event markers sent from the vLP to the aLP. In themaster/slave configuration, AS event markers are sent from the aLP eachtime that an atrial event is sensed outside of the post ventricularatrial blanking (PVAB) interval or some other alternatively-definedatrial blanking period. The AP event markers are sent from the aLP eachtime that the aLP delivers a pacing pulse in the atrium. The aLP mayrestrict transmission of AS markers, whereby the aLP transmits AS eventmarkers when atrial events are sensed both outside of the PVAB intervaland outside the post ventricular atrial refractory period (PVARP) orsome other alternatively-defined atrial refractory period.Alternatively, the aLP may not restrict transmission of AS event markersbased on the PVARP, but instead transmit the AS event marker every timean atrial event is sensed.

TABLE 1 “A2V” Markers/Commands (i.e., from aLP to vLP) MarkerDescription Result in vLP AS Notification of a sensed event in InitiateAV interval (if not atrium (if not in PVAB or PVARP) in PVAB or PVARP)AP Notification of a paced event Initiate PAVB in atrium Initiate AVinterval (if not in PVARP)

As shown in Table 1, when an aLP transmits an event message thatincludes an AS event marker (indicating that the aLP sensed an intrinsicatrial event), the vLP initiates an AV interval timer. If the aLPtransmits an AS event marker for all sensed events, then the vLP wouldpreferably first determine that a PVAB or PVARP interval is not activebefore initiating an AV interval timer. If however the aLP transmits anAS event marker only when an intrinsic signal is sensed outside of aPVAB or PVARP interval, then the vLP could initiate the AV intervaltimer upon receiving an AS event marker without first checking the PVABor PVARP status. When the aLP transmits an AP event marker (indicatingthat the aLP delivered or is about to deliver a pace pulse to theatrium), the vLP initiates a PVAB timer and an AV interval time,provided that a PVARP interval is not active. The vLP may also blank itssense amplifiers to prevent possible crosstalk sensing of the remotepace pulse delivered by the aLP.

TABLE 2 “V2A” Markers/Commands (i.e., from vLP to aLP) MarkerDescription Result in aLP VS Notification of a sensed event in InitiatePVARP ventricle VP Notification of a paced event in Initiate PVABventricle Initiate PVARP AP Command to deliver immediate Deliverimmediate pace pulse pace pulse in atrium to atrium

As shown in Table 2, when the vLP senses a ventricular event, the vLPtransmits an event message including a VS event marker, in response towhich the aLP may initiate a PVARP interval timer. When the vLP deliversor is about to deliver a pace pulse in the ventricle, the vLP transmitsVP event marker. When the aLP receives the VP event marker, the aLPinitiates the PVAB interval timer and also the PVARP interval timer. TheaLP may also blank its sense amplifiers to prevent possible crosstalksensing of the remote pace pulse delivered by the vLP. The vLP may alsotransmit an event message containing an AP command marker to command theaLP to deliver an immediate pacing pulse in the atrium upon receipt ofthe command without delay.

The foregoing event markers are examples of a subset of markers that maybe used to enable the aLP and vLP to maintain full dual chamberfunctionality. In one embodiment, the vLP may perform all dual-chamberalgorithms, while the aLP may perform atrial-based hardware-relatedfunctions, such as PVAB, implemented locally within the aLP. In thisembodiment, the aLP is effectively treated as a remote ‘wireless’ atrialpace/sense electrode. In another embodiment, the vLP may perform mostbut not all dual-chamber algorithms, while the aLP may perform a subsetof diagnostic and therapeutic algorithms. In an alternative embodiment,vLP and aLP may equally perform diagnostic and therapeutic algorithms.In certain embodiments, decision responsibilities may be partitionedseparately to one of the aLP or vLP. In other embodiments, decisionresponsibilities may involve joint inputs and responsibilities.

In an embodiment, ventricular-based pace and sense functionalities arenot dependent on any i2i communication, in order to provide safertherapy. For example, in the event that LP to LP (i2i) communication islost (prolonged or transient), the system 100 may automatically revertto safe ventricular-based pace/sense functionalities as the vLP deviceis running all of the necessary algorithms to independently achievethese functionalities. For example, the vLP may revert to a VVI mode asthe vLP does not depend on i2i communication to perform ventricularpace/sense activities. Once i2i communication is restored, the system100 can automatically resume dual-chamber functionalities.

Messages that are transmitted between LPs (e.g., the aLP and the vLP)can be referred to herein generally as i2i messages, since they areimplant-to-implant messages. As noted above, such messages can includeevent markers that enable one LP to inform the other LP of a paced eventor a sensed event. For example, in certain embodiments, whenever the aLPsenses an atrial event or paces the right atrium, the aLP will transmitan i2i message to the vLP to inform the vLP of the sensed or paced eventin the atrium. In response to receiving such an i2i message, the vLP maystart one or more timers that enable the vLP to sense or pace in theright ventricle. Similarly, the vLP may transmit an i2i message to theaLP whenever the vLP senses a ventricular event or paces the rightventricle.

The i2i messages that are sent between LPs may be relatively shortmessages that simply allow a first LP to inform a second LP of an eventthat was sensed by the first LP or caused (paced) by the first LP, andvice versa. Such i2i messages can be referred to herein as event markeri2i messages, or more succinctly as event i2i messages. The i2i messagesthat are sent between LPs, in certain instances, can be extended i2imessages that include (in addition to an event marker) an extension. Incertain embodiments, an extended i2i message includes an event marker(e.g., 9 bits), followed by an extension indicator (e.g., 2 bits),followed by an extended message payload portion (e.g., 17 bits),followed by a cyclic redundancy check (CRC) code (e.g., 6 bits) or someother type of error detection and correction code.

In certain embodiments, whenever an i2i message is sent by an LP (orother type of IMD, such as a S-ICD), the i2i message will include anextension indicator so that the receiving LP knows whether or not thei2i message it receives includes an extension portion. In suchembodiments, even a relatively short event i2i message will include anextension indicator. The extension indicator (e.g., 2 bits) is used bythe LP (or other IMD) sending the i2i message to indicate, to the LPreceiving the i2i message, whether or not the i2i message is an extendedi2i message. In certain embodiments, if the LP receiving an i2i messagedetermines based on the extension indicator bits that the received i2imessage is not an extended i2i message, then the LP receiving the i2imessage can ignore any bits that follow the extension bits. In such acase, the LP receiving the i2i message only decodes the event marker. Onthe other hand, if the LP receiving an i2i message determines based onthe extension indicator bits that the received i2i message is anextended i2i message, then the LP receiving the i2i message will alsodecode the bits that follow the extension bits, and determine based on aCRC code (or other type of error detection and correction code), whetherthe i2i message is a valid message. If the extended i2i message is avalid i2i message, then the LP receiving the extended i2i message willas appropriate modify its operation, update parameters, and/or the like,based on information included in the extended i2i message. In certainembodiments, event i2i messages that are not extended i2i messages donot include any error detection and correction code.

In an extended i2i message, the event marker bits and the extensionindicator bits are located, respectively, in an event marker field andan extension indicator field of an i2i message packet. In certainembodiments, the extended portion (that follows the event marker bitsand the extension indicator bits) includes message bits (in a messagefield) and rate indicator bits (in a rate indicator field), which areparts of the payload. The payload can alternatively, or additionally,include other types of fields, such as an acknowledgement field that isused in certain situations for one LP to acknowledge reception of an i2imessage from another LP of certain (e.g., critical) types of message.

More generally, various different types of information may be includedwithin the payload of an extended i2i message. For one example, thepayload can include a pacing rate indicator that enables one LP toinform another LP of a pacing rate. For example, assume that an LPsystem provides rate responsive pacing, wherein a pacing rate isadjusted in dependence on a patient's physical activity as detected,e.g., using an accelerometer, temperature sensor, and/or other type ofsensor of an LP. In such an LP system, the vLP may inform the aLP of therate at which the patient's heart should be paced so that the aLP andvLP can perform synchronized pacing. To achieve this, the vLP can send apacing rate indicator to the aLP in the payload of an extended i2imessage. The pacing rate indicator can, e.g., be a value indicating apacing rate value (e.g., 80 bpm), a code that the aLP that can look up(e.g., in a stored look up table) and corresponds to a pacing ratevalue, or a value that the aLP feeds into an equation to determine thepacing rate, but is not limited thereto. Alternatively, the pacing rateindicator can be beat-to-beat interval value (e.g., 0.75 seconds), acode that the aLP can look up and corresponds to a beat-to-beat intervalvalue, or a value that the aLP feeds into an equation to determine thebeat-to-beat interval, but is not limited thereto. Other variations arealso possible and within the scope of the embodiments described herein.

False Messages

As noted above, implantable medical devices and systems often rely onproper communication to operate correctly. For example, in a dualleadless cardiac pacemaker system, such as the one described above withreference to FIGS. 1-5 , i2i communication is critical for propersynchronization of the system. However, noise may cause one or moredevices of such a system to falsely detect an i2i message andinappropriately respond thereto. For example, an atrial LP may falselydetect a message from a ventricular LP, wherein the false messageincludes a portion which the atrial LP mistakenly decodes as a pacingrate indicator that causes the atrial LP to pace the right atrium at aninappropriate high-rate. As also noted above, to reduce the chances offalse message, such messages can include redundant data for errordetection and correction. However, due to the desire to keep the powerconsumption low, the messaging and/or error correction and detectionscheme may be simple and false messages may still get through.

Certain embodiments of the present technology described herein can beused to reduce how often an IMD, such as a vLP (e.g., 102 b) or an aLP(e.g., 102 a), accept false messages. Additionally, or alternatively,certain embodiments of the present technology can be used to mitigatethe adverse effects of an IMD accepting one or more false messages.

When a message is accepted by an IMD, the IMD may trigger a timer,trigger an event and/or otherwise be responsive to the message tocontrol or provide an instruction to the IMD that received the message.By contrast, when a message is rejected, this means that the message isprevented (e.g., blocked) from being used to trigger a timer, trigger anevent and/or otherwise being used to control or provide an instructionto the IMD that received the message.

The term “message”, as used herein, can refer to an actual sent messagethat is received and is capable of being decoded by an IMD, an actualsent message that is received but is too noisy to be decoded by the IMD,an actual sent message that is received but due to noise it is decodedmistakenly for a different message, noise that is received and isinitially mistaken for being an actual message but cannot be decoded bythe IMD because it is sufficiently different than an actual message, aswell as noise that is received and is mistaken for being an actualmessage and is decoded by the IMD because it is sufficiently similarthan an actual message. The term “false message”, as used herein, refersto noise that is received and decoded by the IMD and is mistaken forbeing an actual message because it is sufficiently similar to an actualmessage. The term “false message”, as used herein, can also refer to anactual sent message that is received but due to noise it is decodedmistakenly for a different message. The term “true message”, as usedherein, refers to an actual sent message that is received by an IMD andis correctly decoded by the IMD. An actual sent message may have beensent by another IMD, or alternatively, by a non-implanted device, suchas a programmer (e.g., 109). Where an actual sent message includesmultiple parts, it is possible that a first part of a received messageis correctly decoded and a second part of the received message isincorrectly decoded, in which case it can be said that first part of thereceived message is a “true sub-message” and the second part of thereceived message is a “false sub-message.”

In a system that includes an aLP (e.g., 102 a) and a vLP (e.g., 102 b),which are intended to provide for coordinated (aka synchronized) pacingof atrial and ventricular cardiac chambers, false messages couldpotentially cause the aLP and the vLP to become unsynchronized to thepoint that it takes a significant amount of time for the aLP and vLP tobecome resynchronized, or to the point that they could not becomeresynchronized. For example, if the aLP receives a false message thatinstructs the aLP to pace at a high rate (e.g., 110 bpm), when the vLPis actually pacing at a low rate (e.g., 60 bpm), the aLP will pace theright atrium at a much higher rate than the vLP is pacing the rightventricle. When pacing at the high rate, the aLP will search formessages from the vLP at a high rate, but the vLP will still sendmessages at the low rate. If the rate difference is sufficiently large,there may never be an opportunity for the aLP and the vLP to sync backup, and the aLP and the vLP being out of sync with one another cancontinue indefinitely. In this situation, the vLP device can no longercorrect the incorrect high rate being used by the aLP. In certainimplementations, during this time the system will go into “safe mode”,and the aLP will stop pacing indefinitely. The problem of the aLP andthe vLP not being able to sync back up with one another can be referredto herein as the “lock-up” problem.

The above described “lock-up” problem may occur, e.g., where the aLPreceives a false i2i message that instructs the aLP to pace at a muchhigher rate than the vLP is pacing, or where the vLP receives a falsei2i message that instructs the vLP to pace at a much higher rate thanthe aLP is pacing. The so called “lock-up” problem may also occur forvarious other reasons, discussed below, some of which relate to an LPreceiving a false message, and others of which relate to an LP failingto receive a sent message. For example, if the vLP sends an i2i messageto the aLP that informs the aLP that it should pace at a much higherrate than it had been (and assuming the vLP itself increases its rate tothat much higher rate), if the aLP fails to receive that i2i message andthus does not increase its pacing rate, then the aLP may pace at a ratemuch lower than the vLP, potentially preventing the aLP and the vLP frombeing able to sync back up with one another again.

Examples of other types of i2i message that if missed could potentiallycause the above described “lock-out” problem are i2i messages thatinclude, for example, a recommended replacement time (RRT) indicator, oran automatic mode switch (AMS) entry indicator, an AMS exit indicator, amagnet entry indicator, or a magnet exit indicator. Each of theseindicators are discussed below, along with an explanation of how failingto receive an i2i message including such an indicator could potentiallycause the above described “lock-out” problem.

In accordance with certain embodiments, when the aLP detects atrialflutter (AFI) or atrial fibrillation (AF), the aLP will trigger anautomatic mode switching (AMS). Auto Mode Switching (AMS) is a standarddual-chamber pacemaker feature that, upon detection of a high atrialrate (e.g., during atrial fibrillation or flutter), provides automatictransition from an AV synchronous pacing mode to one without atrialtracking so as to avoid non-physiologically high ventricular rates thatmight otherwise result in adverse/symptomatic hemodynamic cardiacperformance. Conversely, when the high atrial rate reverts to a morephysiologic rate, AMS functionality will terminate and the pacemakersystem will again transition back to an AV synchronous pacing mode.Furthermore, pacemaker systems may utilize these AMS entry and exitevents as triggers to initiate additional actions, such as collectingdiagnostic data, storing intracardiac electrograms, etc.

With the use of two independent LPs (e.g., a vLP and an aLP) operatingin a dual-chamber mode, it may be desirable that the LPs respond to AMSentry and exit events in a consistent and synchronized manner (assumingthat AMS functionality is available and selected). One means toaccomplish this response synchronization is to send a special messagefrom a first LP (e.g., the aLP) to a second LP (e.g., the vLP) thatindicates that the threshold for AMS entry or exit has been met. SinceAMS entry/exit thresholds relate to atrial rates, a preferredimplementation would be to have the aLP be directly responsible fordetermining AMS transitions, with the aLP then communicating thattransition event to the vLP via a special message. Alternatively, thevLP could be responsible for determining AMS transitions via monitoringof the rate at which it receives atrial sense (AS) atrial-to-ventricular(A2V) i2i markers.

Since the underlying high atrial rate could persist for a relativelylong and undetermined duration, an exemplary embodiment includes sendingAMS special messages (e.g., an “AMS Entry” special message) from thefirst LP to the second LP upon reaching the AMS entry trigger, and thensending a separate ‘AMS Exit’ special message upon reaching the AMS exittrigger. In other words, an aLP can send an i2i message that includes anAMS entry indicator to the vLP whenever the aLP enters an AMS mode, andthe aLP can send an i2i message to the vLP whenever the aLP exits theAMS mode. If the vLP receives a false i2i message that includes what thevLP decodes as an AMS entry indicator, the vLP and the aLP may becomeout of sync with one another, potentially resulting in the abovedescribed lock-up problem where the aLP and the vLP are unable to syncback up with one another.

It is likely that the separate and independent LPs will reach theirindividual recommended replacement times (RRT) at different points intheir lifetimes (e.g., due to different initial battery capacities,different pacing output levels or burdens, etc.). However, it may bedesirable or important for the dual-chamber system to reactsynchronously to the realization of RRT by either LP. For example, itmay be desirable to turn off rate-responsiveness upon reaching RRT. Asanother example, it may be desirable to reduce the base rate uponreaching RRT. Modification of other features might also be considered.One means to accomplish this RRT response synchronization is to send aspecial message from a first LP to a second LP that indicates that theRRT threshold has been reached in the first LP. In other words, a firstLP may send an i2i message including an RRT reached indicator to asecond LP when the first LP reaches its RRT. The second LP, in responseto receiving the RRT reached indicator from the first LP, may turn offcertain types of circuitry and/or functionality. If a first LP receivesa false i2i message that includes what the first LP decodes as an RRTreached indicator that it believes was sent by a second LP, then thismay cause the first and second LPs (e.g., the vLP and the aLP) to becomeout of sync with one another, potentially resulting in the abovedescribed lock-out problem.

A magnet externally-applied to a patient that has been implanted with anIMD (e.g., pacemaker, ICD, etc.) is a standard means to (a) immediatelyinitiate non-inhibited fixed-rate pacing (e.g., DOO, VOO, or AOO mode,as appropriate), and/or (b) provide a quick means to assess the IMD'sbattery status (via a standardized pattern of induced pacing rates).With the use of two independent LPs operating in a dual-chamber mode, itis desirable that these LPs respond to an applied magnet in a consistentand synchronized manner (assuming that Magnet Mode functionality isavailable and selected). One means to accomplish this responsesynchronization is to send a special message from a first LP to a secondLP indicating that a magnet has been (or is being) actively detected bythe first LP. In other words, a first LP that detects a magnet can sendan i2i message including a magnet detection indicator to a second LP toindicate initial detection of the magnet by the first LP, and then aseparate i2i message could be sent to indicate loss of detection of thatmagnet by the first LP. Upon detection of the applied magnet by thefirst LP and receipt of the i2i message including the magnet detectionindicator by the second LP, the LPs could immediately and synchronouslyinitiate the appropriate pre-defined or programmed Magnet Mode protocol.For example, the LPs could immediately transition from their programmeddual-chamber functional mode (e.g., DDDR) to the defined non-inhibitedfixed-rate magnet mode (e.g., DOO or VOO). Furthermore, the patternand/or rate of pacing output could conform to the defined magnet modeprotocol (e.g., per AAMI PC88). The magnet mode settings could bemaintained by both LPs until the magnet is no longer detected, at whichpoint the LPs would synchronously revert to their normal mode andfunctionality. If a first LP receives a false i2i message that includeswhat the first LP decodes as a magnet detection indicator that itbelieves was sent by a second LP, then this may cause the first andsecond LPs (e.g., the vLP and the aLP) to become out of sync with oneanother, potentially resulting in the above described lock-out problem.

Various embodiments of the present technology, described herein, can beused to prevent or reduce the probability of the above described“lock-up” problem occurring. Additionally, or alternatively, variousembodiments of the present technology described herein can be used tomitigate the adverse effects of false messages, should they occur. Thesevarious embodiments, described herein, can be used alone or incombination with one another. For example, one, two, or more of thebelow described embodiments can be implemented.

Slew Rate Protection

In accordance with certain embodiments of the present technology, whichare for use with a system including two or more LPs, whenever a first LPreceives an i2i message from a second LP, which message instructs thesecond LP to increases its pacing rate beyond a threshold amount, thesecond LP limits its increase to the threshold amount which is set atsome level that prevents the LPs from getting too far out ofsynchronization with one another. With such embodiments, if there isindeed a need for a pacing rate increase to occur beyond the thresholdamount, the increase would need to occur gradually, rather than all atonce, so as to avoid the above described “lock-up” problem. Suchembodiments can be referred to as the slew rate protection embodimentssince they limit the rate at which one LP may increase its pacing ratein response to a message, which may (or may not) be a false message.Such embodiments can also be used where one or more LPs are configuredto adjust their pacing rate in response to i2i messages including pacingrate indicators transmitted by another type of IMD, such as an S-ICD,but not limited thereto. More generally, such embodiments, which aredescribed in additional detail below with reference to the high levelflow diagram of FIG. 6A, are for use by a leadless pacemaker (LP)implanted in or on a first cardiac chamber of a patient that also has animplantable medical device (IMD) remotely located relative to the LP,wherein the LP is configured to pace the first cardiac chamber (e.g.,the right atrial chamber, or the right ventricular chamber) and adjust apacing rate at which the first cardiac chamber is paced based on apacing rate indicator included in an i2i message received from the IMD(e.g., another LP, or an S-ICD).

Referring to FIG. 6A, step 602 involves the LP monitoring for i2imessages. The LP that performs step 602 can be, e.g., the aLP 102 aimplanted in (or on) the right atrial chamber, but is not limitedthereto. The i2i messages being monitored for at step 602 can be i2imessages that are transmitted by another LP (e.g., the vLP 102 bimplanted in or on the right ventricular chamber) or by an S-ICD (e.g.,106), but is not limited thereto. Such i2i messages that are monitoredfor can include extended i2i messages that include a pacing rateindicator within their payload. One or more receivers (e.g., 120 and/or122) of an LP can be used to perform step 602. Such receiver(s) can beconnected to electrodes (e.g., 108) where the i2i messages areconductive communication type messages, or can be connected to anantenna (e.g., 128) where the i2i messages are RF communication typemessages.

Still referring to FIG. 6A, at step 604 there is a determination ofwhether an i2i message is received. If an i2i message is determined tohave not been received, then flow returns to step 602 and the LPcontinues to monitor for an i2i message. If an i2i message is determinedto have been received, then flow goes to step 606. The message that isdetermined to have been received at step 604 may not actually be a truemessage, but rather, may be a false message.

The types of messages that may be received include relatively simpleevent marker i2i messages that do not include a pacing rate indicator,or extended i2i messages that may include a pacing rate indicator. Asnoted above, such extended i2i messages may include error detection andcorrection codes, such as CRC codes. Accordingly, step 602 and/or 604can involve performing error detection and correction.

At step 606 there is a determination of whether the received i2i messageincludes a pacing rate indicator. As noted above, a pacing rateindicator can, e.g., be a value indicating a pacing rate value (e.g., 80bpm), a code that an LP can look up (e.g., in a stored look up table)and corresponds to a pacing rate value, or a value that an LP feeds intoan equation to determine the pacing rate, but is not limited thereto.Alternatively, the pacing rate indicator can be beat-to-beat intervalvalue (e.g., 0.75 seconds), a code that the LP can look up andcorresponds to a beat-to-beat interval value, or a value that the LPfeeds into an equation to determine the beat-to-beat interval, but isnot limited thereto. Other variations are also possible and within thescope of the embodiments described herein. For another example, thepacing rate indicator can be a signed adjustment value or code thatspecifies how much the LP should increase its pacing rate (if the signedadjustment value or code has a positive sign) or decrease its pacingrate (if the signed adjustment value or code has a negative sign). Wherean LP is to adjust its pacing rate to be equal to a new rate indicatedby another LP or other type of IMD, rather than jump right to the newrate, the LP may gradually adjust its pacing rate, e.g., linearly,exponentially, or in some other manner.

If the answer the determination at step 606 is No (meaning that thereceived i2i message does not include a pacing rate indicator), thenflow goes to step 610 and there is no adjustment to the pacing rate. Ifthe answer the determination at step 606 is Yes (meaning that thereceived i2i message does include a pacing rate indicator), then flowgoes to step 608 and there is a determination as to whether adjustingthe pacing rate to match the rate specified by the pacing rate indicatorwould cause an adjustment to the pacing rate to exceed a rate adjustmentthreshold. The rate adjustment threshold can be a predetermined value(e.g., 15 bpm, or 20 bpm). Alternatively, the rate adjustment thresholdcan be a predetermined function of the present pacing rate. For example,the rate adjustment threshold can be a predetermined percentage (e.g.,15% or 20%) of the present pacing rate. For another example, the rateadjustment threshold can be a predetermined percentage (e.g., 25%) of adifference between the present pacing rate (e.g., 80 bpm) and a basepacing rate (e.g., 60 bpm). For a further example, the rate adjustmentthreshold can be limited to the greater of (or the lesser of) apredetermined value, a predetermine percentage of the present pacingrate, or a predetermined percentage of a difference between the presentpacing rate and a base pacing rate. Other variations are also possibleand within the scope of the embodiments described herein. Further, it isnoted that a rate adjustment threshold that is used when a pacing rateis being increased can differ from a rate adjustment threshold that isused when a pacing rate is being decreased. In other words, there can bea rate increase threshold and a rate decrease threshold, which maydiffer from one another.

If the answer the determination at step 608 is No (meaning thatadjusting the pacing rate to match the rate specified by the pacing rateindicator would not cause the adjustment to the pacing rate to exceedthe rate adjustment threshold), then flow goes to step 612. At step 612an adjustment to the pacing rate is made to match the rate specified bythe pacing rate indicator).

If the answer the determination at step 608 is Yes (meaning thatadjusting the pacing rate to match the rate specified by the pacing rateindicator would cause the adjustment to the pacing rate to exceed therate adjustment threshold), then flow goes to step 614. At step 614 thepacing rate is adjusted, but the amount by which the pacing rate isadjusted is limited to a specified amount. The specified amount (whichthe pacing rate adjustment is limited to) can be a predetermined value(e.g., 15 bpm or 20 bpm), or a predetermined function of the presentpacing rate, but is not limited thereto. For example, the predeterminedamount can be a predetermined percentage (e.g., 15% or 20%) of thepresent pacing rate. For another example, the predetermined amount canbe a predetermined percentage (e.g., 25%) of a difference between thepresent pacing rate (e.g., 80 bpm) and a base pacing rate (e.g., 60bpm). Other variations are also possible and within the scope of theembodiments described herein.

Periodic Reduction in Pacing Rate

Assume the vLP acts as a “master” and the aLP acts as a “slave” in amaster/slave leadless pacemaker system configuration. As noted above,“lock-up” may occur, for example, if the aLP receives a false messagethat instructs the aLP to pace at a high rate, when the vLP is actuallypacing at a low rate, thereby causing aLP to pace the atrium at a muchhigher rate than the vLP is pacing the ventricle. To avoid the aLP fromremaining out of sync with the vLP indefinitely, whenever the aLP doesnot receive an i2i message from the vLP for at least a specified period,the aLP will periodically (e.g., once per specified length of time, oronce per specified number of cardiac cycles) reduce its pacing rate by aspecified amount (e.g., value or percentage), thereby eventually causingthe pacing rate aLP to get close enough to the pacing rate of the vLPsuch that the aLP can receive i2i messaged from the vLP and the aLP andthe vLP can get back in sync with one another. Such embodiments can alsobe used where one or more LPs are configured to adjust their pacing ratein response to i2i messages including pacing rate indicators transmittedby another type of IMD, such as an S-ICD, but not limited thereto. Moregenerally, such embodiments, which are described in additional detailbelow with reference to the high level flow diagram of FIG. 6B, are foruse by a leadless pacemaker (LP) implanted in or on a first cardiacchamber of a patient that also has an implantable medical device (IMD)remotely located relative to the LP, wherein the LP is configured topace the first cardiac chamber (e.g., the right atrial chamber, or theright ventricular chamber) and adjust a pacing rate at which the firstcardiac chamber is paced based on a pacing rate indicator included in ani2i message received from the IMD (e.g., another LP, or an S-ICD).

Referring to FIG. 6B, step 602 involves the LP monitoring for i2imessages. Step 602 in FIG. 6B is the same as step 602 described abovewith reference to FIG. 6A, and thus, need not be described again. Atstep 603 there is a determination of whether a specified period (withinwhich an i2i message that includes a pacing rate indicator is expectedto be received, or within which specified a number of i2i message thatinclude a pacing rate indicator is expected to be received) has expired.The specified period, which can also be referred to as an expectedperiod, can be a predetermined period of time, e.g., 1 second, 1.5seconds, 2 seconds, 5 seconds, or 10 seconds, but is not limitedthereto. The expected period (aka specified period) can alternatively bea specified number (N) of cardiac cycles, where N is a predeterminedinteger that is equal to or greater than 1. For example, N can be 1, 2,3, 5, 10, or 15, but is not limited thereto.

If the answer to the determination at step 603 is No, then flow goes tostep 604. At step 604 there is a determination of whether an i2i messageis received. If an i2i message is determined to have not been received,then flow returns to step 602 and the LP continues to monitor for an i2imessage. If an i2i message is determined to have been received, thenflow goes to step 606. Steps 604 and 606 are the same as steps 604 and606 described above with reference to FIG. 6A, and thus, need not bedescribed again. Additionally, steps 606, 608, 610, 612, and 614 in FIG.6B are the same as those commonly numbered steps described above withreference 6A, and thus, need to be described again. In an alternativeembodiment, the order of steps 603 and 604 are reversed, flow would gofrom step 604 to 603 if the answer to the determination at step 604 wasNo, flow would go from step 603 back to step 602 if the answer to step603 was No, if the answer to step 603 was Yes flow would still go tostep 605, and if the answer to step 604 was Yes flow would still go tostep 606. In FIG. 6B, it would also be possible for flow to jumpdirectly from step 606 to 612 in FIG. 6B, if the answer to thedetermination at step 606 is Yes. Other variations are also possible.

Returning to step 603 in FIG. 6B, if the answer to the determination atstep 603 is Yes (meaning that a specified period within which an i2imessage is expected to have been received has expired), then flow goesto step 605. At step 605, in response to the LP not receiving an i2imessage within the expected period, the LP reduces its pacing rate atwhich it paces the cardiac chamber (e.g., a first cardiac chamber) it isresponsible for pacing. At step 605 the amount by which the LP reducesthe pacing rate at which the LP paces the first cardiac chamber, inresponse to the LP not receiving an i2i message within an expectedperiod, can be a predetermined value, e.g., 5 bpm, 10 bpm, or 15 bpm,but is not limited thereto. Alternatively, at step 605 the amount bywhich the LP reduces the pacing rate can be a predetermined function ofa present pacing rate. For example, the amount can be a predeterminedpercentage (e.g., 15% or 20%) of the present pacing rate. For anotherexample, the amount can be a predetermined percentage (e.g., 25%) of adifference between the present pacing rate (e.g., 80 bpm) and a basepacing rate (e.g., 60 bpm). Other variations are also possible andwithin the scope of the embodiments described herein.

Multiple Messages

In accordance with certain embodiments of the present technology, when asecond LP (or other type of IMD, e.g., an S-ICD) sends certain types ofextended i2i messages to a first LP, the second LP (or other type ofIMD) must send the extended i2i message at least M times (where M is aninteger that is greater than or equal to 2) within a specified amount oftime or cardiac cycles, and the first LP must receive the extended i2imessage at least N times (where N is an integer that is greater than orequal to 2, and can be equal to or less than M) within the specifiedamount of time or cardiac cycles, in order for the first LP to followinstructions included in the extended i2i message. Exemplary types ofextended i2i messages that a sending LP (or other type of IMD) must sendat least M times, and the receiving LP must receive at least N times,can include, but are not limited to, an extended i2i message thatincludes at least one of a pacing rate indicator, a recommendedreplacement time (RRT) indicator, or an automatic mode switch (AMS)entry indicator, an AMS exit indicator, a magnet entry indicator, or amagnet exit indicator, but is not limited thereto.

The high level flow diagram of FIG. 6C will now be used to summarizemethods in which multiple i2i messages including a pacing rate indicatormust be sent to and received by an LP before the LP adjusts its pacingrate in response to receiving i2i messages including the pacing rateindicator. Such methods are for use by an LP implanted in or on a firstcardiac chamber of a patient that also has an IMD remotely locatedrelative to the LP, wherein the LP is configured to pace the firstcardiac chamber and adjust a pacing rate at which the first cardiacchamber is paced based on a pacing rate indicator included in an i2imessage received from the IMD. The IMD can be another LP implanted in oron a second cardiac chamber, or an S-ICD, but is not limited thereto.While FIG. 6C is described from the perspective of the LP that receivesi2i messages (including pacing rate indicators) from another IMD (e.g.,another LP), it should be understood that the IMD (e.g., another LP)that transmits the i2i messages (including pacing rate indicators)should be configured to transmit at least M consecutive i2i messagesthat include a pacing rate indicator whenever the IMD wants to cause theLP (to which the i2i messages are being sent) to change its pacing rate.

Referring to FIG. 6C, step 602 involves an LP monitoring for i2imessages, step 604 involves determining whether an i2i message wasreceived, and step 606 involves determining whether a received i2imessage includes a pacing rate indicator. Steps 602, 604, and 606 arethe same as the commonly numbered steps described above with referenceto FIG. 6A, and thus, need not be described again.

If the answer the determination at step 606 is No (meaning that thereceived i2i message does not include a pacing rate indicator), thenflow goes to step 610 and there is no adjustment to the pacing rate. Ifthe answer the determination at step 606 is Yes (meaning that thereceived i2i message does include a pacing rate indicator), then flowgoes to step 618. At step 618 there is a determination of whether theprevious N−1 i2i messages received also included the same pacing rateindicator. More generally, at step 602, 604, 606, and 618 there is adetermination of whether N consecutive received i2i messages include thesame pacing rate indicator, where N is an integer that is equal to orgreater than 2. If the answer to the determination at step 618 is No,then flow goes to step 610 and there is no adjustment to the pacingrate. If the answer the determination at step 618 is Yes (meaning that Nconsecutive received i2i messages included the same pacing rateindicator), then flow goes to step 622. At step 622 the pacing rate isadjusted to based on the pacing rate indicator included in the Nconsecutive received i2i messages.

As noted above, a pacing rate indicator can, e.g., be a value indicatinga pacing rate value (e.g., 80 bpm), a code that the LP that can look up(e.g., in a stored look up table) and corresponds to a pacing ratevalue, or a value that the LP feeds into an equation to determine thepacing rate, but is not limited thereto. Alternatively, the pacing rateindicator can be beat-to-beat interval value (e.g., 0.75 seconds), acode that the LP can look up and corresponds to a beat-to-beat intervalvalue, or a value that the LP feeds into an equation to determine thebeat-to-beat interval, but is not limited thereto. Other variations arealso possible and within the scope of the embodiments described herein.

With respect to the embodiments described with reference to FIG. 6C, byrequiring that an LP receive N consecutive i2i messages including thesame pacing rate indicator, in order for the LP to change its pacingrate based on the pacing rate indicator, the probability that an LPadjusts its pacing rate in response to a false message is significantlyreduced, which also has the effect of significantly reducing theprobability one or more false messages will cause the above describedlock-up problem. In other words, such embodiments take advantage ofthere being a very low probability that an LP receives multipleconsecutive false i2i messages including a same pacing rate indicator.In certain alternative embodiments, rather than requiring that Nconsecutive i2i messages include the same pacing rate indicator, inorder for an LP to change its pacing rate based on the pacing rateindicator, the LP can change its pacing rate so long as M out of Nreceived i2i messages include the same pacing rate indicator, wherein Mis a first specified integer that is 2 or greater, and N is a secondspecified integer that is greater than M (e.g., M=3 and N=5).

Selective Increased Error Detection and Correction Code Length

An LP may use cyclic redundancy check (CRC) or some other type of errordetection and correction scheme to determine whether a message the LPreceives is a valid message or an invalid message. The shorter a messageis, the greater the probability that an LP may receive a “falsemessage”. Conversely, the longer a message is, the lower the probabilitythat an LP may receive a “false message”. However, using longer messagesconsumes more power than using shorter messages, and thus, it would notbe practical from a device longevity standpoint for every message sentbetween LPs and/or other types of IMDs to be long messages.

Within an error detection and correction scheme, error detectiongenerally refers to the detection of errors caused by noise or otherimpairments during transmission from a transmitter of one device to areceiver of another device. Error correction generally refers to thedetection of errors and reconstruction of the original, error-free data,if possible. Typically, to enable error detection and correction to beperformed, some redundancy (i.e., some extra data) is added to amessage, which enables a receiver to check consistency of the receivedmessage, and to recover data determined to be corrupted. Error detectionis often realized using a suitable hash function (or checksum algorithm)that adds a fixed-length tag to a message, which enables receivers toverify the delivered message by recomputing the tag and comparing itwith the one provided. For example, a repetition code can be used, wherea repetition code is a coding scheme that repeats the bits across achannel to attempt to achieve error-free communication. Such arepetition code is often inefficient, and can be susceptible to problemsif the error occurs in exactly the same place for each group. However,an advantage of repetition codes is that they are extremely simple, andthus, are typically power efficient compared to more complex schemes.Instead of, or in addition to a repetition code, parity bits can beused, wherein a parity bit is a bit that is added to a group of sourcebits to ensure that a number of set bits (e.g., bits with value 1) inthe outcome is even or odd. Alternatively, or additionally, checksumsand/or cyclic redundancy checks can be utilized. A checksum of a messageis a modular arithmetic sum of message code words of a fixed word length(e.g., byte values). The sum may be negated by means of aones'-complement operation prior to transmission to detect errorsresulting in all-zero messages. Checksum schemes can include paritybits, check digits, and longitudinal redundancy checks. A cyclicredundancy check (CRC) is a non-secure hash function designed to detectaccidental changes to digital data.

Where an error is detected in a received message, such an error mayoften be corrected. Such error correction may involve the use of anautomatic repeat request, an error-correcting code or a hybrid scheme,but is not limited thereto. Automatic repeat request (ARQ) is an errorcontrol technique for data transmission that makes use oferror-detection codes, acknowledgment and/or negative acknowledgmentmessages, and timeouts to achieve reliable data transmission. Anacknowledgment is a message sent by the receiver to indicate that it hascorrectly received a data frame. Usually, when a transmitter does notreceive the acknowledgment before the timeout occurs (e.g., within areasonable amount of time after sending the data frame), it retransmitsthe frame until it is either correctly received or the error persistsbeyond a predetermined number of retransmissions. An error-correctingcode (ECC) or forward error correction (FEC) code is a process of addingredundant data, or parity data, to a message, such that it can berecovered by a receiver even when a number of errors (up to thecapability of the code being used) were introduced, either during theprocess of transmission, or on storage. Since the receiver does not haveto ask the sender for retransmission of the data, a backchannel is notrequired in forward error correction, and it is therefore suitable forsimplex communication such as broadcasting. Hybrid ARQ is a combinationof ARQ and forward error correction. The above description has beenincluded to provide a high level of possible error correction anddetection schemes, and is not intended to be limiting and/or allencompassing, as the embodiments of the present technology can be usedwith almost any already developed or future developed error correctionand detection schemes.

While there exist other types of error detection and correction schemesbesides CRC schemes, for much of the discussion herein it is assumedthat a CRC scheme is used. Nevertheless, it should be clear theembodiments of the present technology can be used with other types oferror detection and correction schemes besides CRC. When using a CRCscheme, the CRC is computed from a received message. The receivedmessage plus the CRC must match in order for the combined message to beconsidered valid. The more bits used for the CRC the less likely randomnoise will create a pattern that is coincidentally a message with amatching CRC. It's like adding more digits to a combination lock. Byincreasing the message length of certain types of messages, theprobability of an LP receiving a “false message” that is one of thosecertain types of messages (e.g., a message including a pacing rateindicator, or a message including a critical message, but not limitedthereto) is significantly reduced. By limiting the use of the longermessages to just certain types of messages, the increase in powerconsumption that coincides with using longer messages is limited.Exemplary types of messages with which this solution can be used includemessages that include a pacing rate indicator, a recommended replacementtime (RRT) indicator, an automatic mode switch (AMS) entry or exitindicator, or a magnet entry or exit indicator, and/or the like.

The high level flow diagram of FIG. 6D will now be used to summarizecertain methods for use by an implantable system that includes an LPimplanted in or on a first cardiac chamber of a patient and an IMDremotely located relative to the LP, wherein the LP is configured topace the first cardiac chamber and adjust a pacing rate at which thefirst cardiac chamber is paced based on a pacing rate indicator includedin an i2i message received from the IMD. The IMD can be another LP, oran ICD, such as an S-ICD, but is not limited thereto. Such embodimentsare especially useful where an LP or an S-ICD is acting as a masterdevice for an LP that is acting as a slave device. The flow diagram ofFIG. 6D is described from the perspective of the IMD that istransmitting i2i messages to an LP that receives i2i messages and mayadjust its pacing rate if a received i2i message includes a pacing rateindicator.

Referring to FIG. 6D, at step 632 an IMD determines whether there is anevent (e.g., a sensed or paced event) or a trigger in response to whichthe IMD should send a message to a remotely located LP. For example, ifthe IMD is a vLP, the vLP may send an i2i message to a remotely locatedaLP whenever the vLP paces the right ventricle, or detects an intrinsicventricular event. For another example, if the IMD is an S-ICD, inresponse to detecting a tachycardiac or some other event or condition,the S-ICD may send an i2i message to a vLP that instructs the vLP todeliver anti-tachycardia pacing (ATP). If the answer to step 632 is No,then step 632 is repeated until the answer to step 632 is Yes, at whichpoint flow goes to step 634.

At step 634 the IMD (or more specifically, a controller thereof)determines whether there is information to include in the message suchthat an extended i2i message should be sent. If the answer to thedetermination at step 634 is No, then flow goes to step 638. At step 638an event marker i2i message (which is not an extended message) isgenerated and transmitted.

If the answer to the determination at step 634 is Yes, then flow goes tostep 636. At step 636 there is a determination of whether the extendedi2i message should include a rate adjustment indicator and/or any othertype of indicator (e.g., an RRT indicator, magnet entry indicator,magnet exit indicator, AMS entry indicator, AMS exit indicator, an ATPtrigger, or a store EGM data trigger) for which a longer CRC code is tobe used. The IMD may have a list or table of such message for which alonger CRC code should be used. If the answer to the determination atstep 636 is No, then flow goes to step 642, where an extended i2imessage including a shorter CRC code (e.g., a 4 bit CRC code) isgenerated and transmitted. If the answer to the determination at step636 is Yes, then flow goes to step 640, where an extended i2i messageincluding a longer CRC code (e.g., a 6 bit CRC code) is generated andtransmitted. Other types of error correction and detection codes canalternatively be used in place of CRC codes.

The i2i messages transmitted at instances of steps 638, 640, and 642, orat least a subset of those instances, will be received by an LP to whichthe i2i messages are sent. The LP monitors for i2i messages, and inresponse to receiving an i2i message including a pacing rate indicatormay adjust the rate at which the LP paces the cardiac chamber in (or on)which it is located. In such embodiments, i2i messages including pacingrate indicators that are transmitted from the IMD to the LP includelonger error detection and correction codes compared to at least some ofthe i2i messages not including pacing rate indicators that aretransmitted from the IMD to the LP. The use of longer error detectionand correction codes (e.g., longer CRC codes) for certain types ofmessages reduces the probability that false messages for such types ofmessages will be received, which also has the effect of significantlyreducing the probability the one or more false messages will cause theabove described lock-up problem.

The embodiments described above with reference to FIGS. 6A, 6B, 6C, and6C may be used alone or in combination with one another. For example, anembodiment described with reference to FIG. 6A can be used alone or withone or more of the embodiments described with reference to FIGS. 6B, 6C,and/or 6D. For another example, an embodiment described with referenceto FIG. 6B can be used alone or in with one or more of the embodimentsdescribed with reference to FIGS. 6A, 6C, and/or 6D. For a furtherexample, the embodiments described with reference to FIG. 6C can be usedalone or in combination with one or more of the embodiments describedwith reference to FIGS. 6A, 6B and/or 6D. For still another example, theembodiment described with reference to FIG. 6D can be used alone or incombination with one or more of the embodiment described with referenceto FIGS. 6A, 6B and/or 6C.

FIG. 7 shows a block diagram of one embodiment of an IMD (e.g., an LP orICD) 701 that is implanted into the patient as part of the implantablecardiac system in accordance with certain embodiments herein. IMD 701may be implemented as a full-function biventricular pacemaker, equippedwith both atrial and ventricular sensing and pacing circuitry for fourchamber sensing and stimulation therapy (including both pacing and shocktreatment). Optionally, IMD 701 may provide full-function cardiacresynchronization therapy. Alternatively, IMD 701 may be implementedwith a reduced set of functions and components. For instance, the IMDmay be implemented without ventricular sensing and pacing.

IMD 701 has a housing 700 to hold the electronic/computing components.Housing 700 (which is often referred to as the “can”, “case”,“encasing”, or “case electrode”) may be programmably selected to act asthe return electrode for certain stimulus modes. Housing 700 may furtherinclude a connector (not shown) with a plurality of terminals 702, 704,706, 708, and 710. The terminals may be connected to electrodes that arelocated in various locations on housing 700 or elsewhere within andabout the heart. IMD 701 includes a programmable microcontroller 720that controls various operations of IMD 701, including cardiacmonitoring and stimulation therapy. Microcontroller 720 includes amicroprocessor (or equivalent control circuitry), RAM and/or ROM memory,logic and timing circuitry, state machine circuitry, and I/O circuitry.

IMD 701 further includes a first pulse generator 722 that generatesstimulation pulses for delivery by one or more electrodes coupledthereto. Pulse generator 722 is controlled by microcontroller 720 viacontrol signal 724. Pulse generator 722 may be coupled to the selectelectrode(s) via an electrode configuration switch 726, which includesmultiple switches for connecting the desired electrodes to theappropriate I/O circuits, thereby facilitating electrodeprogrammability. Switch 726 is controlled by a control signal 728 frommicrocontroller 720.

In the embodiment of FIG. 7 , a single pulse generator 722 isillustrated. Optionally, the IMD may include multiple pulse generators,similar to pulse generator 722, where each pulse generator is coupled toone or more electrodes and controlled by microcontroller 720 to deliverselect stimulus pulse(s) to the corresponding one or more electrodes.

Microcontroller 720 is illustrated as including timing control circuitry732 to control the timing of the stimulation pulses (e.g., pacing rate,atrio-ventricular (AV) delay, atrial interconduction (A-A) delay, orventricular interconduction (V-V) delay, etc.). Timing control circuitry732 may also be used for the timing of refractory periods, blankingintervals, noise detection windows, evoked response windows, alertintervals, marker channel timing, and so on. Microcontroller 720 alsohas an arrhythmia detector 734 for detecting arrhythmia conditions and amorphology detector 736. Although not shown, the microcontroller 720 mayfurther include other dedicated circuitry and/or firmware/softwarecomponents that assist in monitoring various conditions of the patient'sheart and managing pacing therapies.

IMD 701 is further equipped with a communication modem(modulator/demodulator) 740 to enable wireless communication with theremote slave pacing unit. Modem 740 may include one or more transmittersand two or more receivers as discussed herein in connection with FIG. 2. In one implementation, modem 740 may use low or high frequencymodulation. As one example, modem 740 may transmit i2i messages andother signals through conductive communication between a pair ofelectrodes. Modem 740 may be implemented in hardware as part ofmicrocontroller 720, or as software/firmware instructions programmedinto and executed by microcontroller 720. Alternatively, modem 740 mayreside separately from the microcontroller as a standalone component.

IMD 701 includes a sensing circuit 744 selectively coupled to one ormore electrodes, that perform sensing operations, through switch 726 todetect the presence of cardiac activity in the right chambers of theheart. Sensing circuit 744 may include dedicated sense amplifiers,multiplexed amplifiers, or shared amplifiers. It may further employ oneor more low power, precision amplifiers with programmable gain and/orautomatic gain control, bandpass filtering, and threshold detectioncircuit to selectively sense the cardiac signal of interest. Theautomatic gain control enables the unit to sense low amplitude signalcharacteristics of atrial fibrillation. Switch 726 determines thesensing polarity of the cardiac signal by selectively closing theappropriate switches. In this way, the clinician may program the sensingpolarity independent of the stimulation polarity.

The output of sensing circuit 744 is connected to microcontroller 720which, in turn, triggers or inhibits the pulse generator 722 in responseto the presence or absence of cardiac activity. Sensing circuit 744receives a control signal 746 from microcontroller 720 for purposes ofcontrolling the gain, threshold, polarization charge removal circuitry(not shown), and the timing of any blocking circuitry (not shown)coupled to the inputs of the sensing circuitry.

In the embodiment of FIG. 7 , a single sensing circuit 744 isillustrated. Optionally, the IMD may include multiple sensing circuits,similar to sensing circuit 744, where each sensing circuit is coupled toone or more electrodes and controlled by microcontroller 720 to senseelectrical activity detected at the corresponding one or moreelectrodes. Sensing circuit 744 may operate in a unipolar sensingconfiguration or in a bipolar sensing configuration.

IMD 701 further includes an analog-to-digital (A/D) data acquisitionsystem (DAS) 750 coupled to one or more electrodes via switch 726 tosample cardiac signals across any pair of desired electrodes. Dataacquisition system 750 is configured to acquire intracardiac electrogramsignals, convert the raw analog data into digital data, and store thedigital data for later processing and/or telemetric transmission to anexternal device 754 (e.g., a programmer, local transceiver, or adiagnostic system analyzer). Data acquisition system 750 is controlledby a control signal 756 from the microcontroller 720.

Microcontroller 720 is coupled to a memory 760 by a suitabledata/address bus. The programmable operating parameters used bymicrocontroller 720 are stored in memory 760 and used to customize theoperation of IMD 701 to suit the needs of a particular patient. Suchoperating parameters define, for example, pacing pulse amplitude, pulseduration, electrode polarity, rate, sensitivity, automatic features,arrhythmia detection criteria, and the amplitude, waveshape and vectorof each shocking pulse to be delivered to the patient's heart withineach respective tier of therapy.

The operating parameters of IMD 701 may be non-invasively programmedinto memory 760 through a telemetry circuit 764 in telemetriccommunication via communication link 766 with external device 754.Telemetry circuit 764 allows intracardiac electrograms and statusinformation relating to the operation of IMD 701 (as contained inmicrocontroller 720 or memory 760) to be sent to external device 754through communication link 766.

IMD 701 can further include magnet detection circuitry (not shown),coupled to microcontroller 720, to detect when a magnet is placed overthe unit. A magnet may be used by a clinician to perform various testfunctions of IMD 701 and/or to signal microcontroller 720 that externaldevice 754 is in place to receive or transmit data to microcontroller720 through telemetry circuits 764.

IMD 701 can further include one or more physiological sensors 770. Suchsensors are commonly referred to as “rate-responsive” sensors becausethey are typically used to adjust pacing stimulation rates according tothe exercise state of the patient. However, physiological sensor 770 mayfurther be used to detect changes in cardiac output, changes in thephysiological condition of the heart, or diurnal changes in activity(e.g., detecting sleep and wake states). Signals generated byphysiological sensors 770 are passed to microcontroller 720 foranalysis. Microcontroller 720 responds by adjusting the various pacingparameters (such as rate, AV Delay, V-V Delay, etc.) at which the atrialand ventricular pacing pulses are administered. While shown as beingincluded within IMD 701, physiological sensor(s) 770 may be external toIMD 701, yet still be implanted within or carried by the patient.Examples of physiologic sensors might include sensors that, for example,sense respiration rate, pH of blood, ventricular gradient, activity,position/posture, minute ventilation (MV), and so forth.

A battery 772 provides operating power to all of the components in IMD701. Battery 772 is capable of operating at low current drains for longperiods of time, and is capable of providing high-current pulses (forcapacitor charging) when the patient requires a shock pulse (e.g., inexcess of 2 A, at voltages above 2 V, for periods of 10 seconds ormore). Battery 772 also desirably has a predictable dischargecharacteristic so that elective replacement time can be detected. As oneexample, IMD 701 employs lithium/silver vanadium oxide batteries.

IMD 701 further includes an impedance measuring circuit 774, which canbe used for many things, including: lead impedance surveillance duringthe acute and chronic phases for proper lead positioning ordislodgement; detecting operable electrodes and automatically switchingto an operable pair if dislodgement occurs; measuring respiration orminute ventilation; measuring thoracic impedance for determining shockthresholds; detecting when the device has been implanted; measuringstroke volume; and detecting the opening of heart valves; and so forth.Impedance measuring circuit 774 is coupled to switch 726 so that anydesired electrode may be used. In this embodiment IMD 701 furtherincludes a shocking circuit 780 coupled to microcontroller 720 by adata/address bus 782.

In some embodiments, the LPs 102 a and 102 b are configured to beimplantable in any chamber of the heart, namely either atrium (RA, LA)or either ventricle (RV, LV). Furthermore, for dual-chamberconfigurations, multiple LPs may be co-implanted (e.g., one in the RAand one in the RV, one in the RV and one in the coronary sinus proximatethe LV). Certain pacemaker parameters and functions depend on (orassume) knowledge of the chamber in which the pacemaker is implanted(and thus with which the LP is interacting; e.g., pacing and/orsensing). Some non-limiting examples include sensing sensitivity, anevoked response algorithm, use of AF suppression in a local chamber,blanking & refractory periods, etc. Accordingly, each LP needs to knowan identity of the chamber in which the LP is implanted, and processesmay be implemented to automatically identify a local chamber associatedwith each LP.

Processes for chamber identification may also be applied to subcutaneouspacemakers, ICDs, with leads and the like. A device with one or moreimplanted leads, identification and/or confirmation of the chamber intowhich the lead was implanted could be useful in several pertinentscenarios. For example, for a DR or CRT device, automatic identificationand confirmation could mitigate against the possibility of the clinicianinadvertently placing the V lead into the A port of the implantablemedical device, and vice-versa. As another example, for an SR device,automatic identification of implanted chamber could enable the deviceand/or programmer to select and present the proper subset of pacingmodes (e.g., AAI or VVI), and for the IPG to utilize the proper set ofsettings and algorithms (e.g., V-AutoCapture vs ACap-Confirm, sensingsensitivities, etc.).

While many of the embodiments of the present technology described abovehave been described as being for use with LP type IMDs, embodiments ofthe present technology that are for use in reducing how often a firstreceiver of an IMD wakes up a second receiver of an IMD, in order toreduce power consumption, can also be used with other types of IMDsbesides an LP. Accordingly, unless specifically limited to use with anLP, the claims should not be limited to use with LP type IMDs.

It is to be understood that the subject matter described herein is notlimited in its application to the details of construction and thearrangement of components set forth in the description herein orillustrated in the drawings hereof. The subject matter described hereinis capable of other embodiments and of being practiced or of beingcarried out in various ways. Also, it is to be understood that thephraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having” and variations thereof herein ismeant to encompass the items listed thereafter and equivalents thereofas well as additional items. Further, it is noted that the term “basedon” as used herein, unless stated otherwise, should be interpreted asmeaning based at least in part on, meaning there can be one or moreadditional factors upon which a decision or the like is made. Forexample, if a decision is based on the results of a comparison, thatdecision can also be based on one or more other factors in addition tobeing based on results of the comparison.

It is to be understood that the above description is intended to beillustrative, and not restrictive. For example, the above-describedembodiments (and/or aspects thereof) may be used in combination witheach other. In addition, many modifications may be made to adapt aparticular situation or material to the teachings of the embodiments ofthe present technology without departing from its scope. While thedimensions, types of materials and coatings described herein areintended to define the parameters of the embodiments of the presenttechnology, they are by no means limiting and are exemplary embodiments.Many other embodiments will be apparent to those of skill in the artupon reviewing the above description. The scope of the embodiments ofthe present technology should, therefore, be determined with referenceto the appended claims, along with the full scope of equivalents towhich such claims are entitled. In the appended claims, the terms“including” and “in which” are used as the plain-English equivalents ofthe respective terms “comprising” and “wherein.” Moreover, in thefollowing claims, the terms “first,” “second,” and “third,” etc. areused merely as labels, and are not intended to impose numericalrequirements on their objects. Further, the limitations of the followingclaims are not written in means—plus-function format and are notintended to be interpreted based on 35 U.S.C. § 112(f), unless and untilsuch claim limitations expressly use the phrase “means for” followed bya statement of function void of further structure.

What is claimed is:
 1. A method for use by a leadless pacemaker (LP)implanted in or on a cardiac chamber of a patient that also has animplantable medical device (IMD) remotely located relative to the LP,wherein the LP is configured to pace the cardiac chamber, the methodcomprising: the LP monitoring for implant-to-implant (i2i) messagestransmitted by the IMD remotely located relative to the LP; and inresponse to the LP receiving at least a specified plurality of i2imessages including a same indicator, the LP making a change based on thesame indicator included in the specified plurality of i2i messagesreceived by the LP; wherein if the LP had not received at least thespecified plurality of i2i messages including the same indicator, the LPwould not make the change.
 2. The method of claim 1, wherein: thespecified plurality of i2i messages comprise at least N consecutive i2imessages including the same indicator, where N is a predeterminedinteger that is equal to or greater than 2; and the method alsocomprises the IMD sending at least N consecutive i2i messages includingthe same indicator to the LP when the IMD makes a specific change to itsoperation that the LP should be informed of, or when the IMD instructsthe LP to make a specific change.
 3. The method of claim 2, wherein: theindicator comprises a pacing rate indicator; the specified plurality ofi2i messages comprise at least N consecutive i2i messages including thesame pacing rate indicator; and the IMD sending comprises the IMDsending at least N consecutive i2i messages including the same pacingrate indicator to the LP when the IMD instructs the LP to change apacing rate.
 4. The method of claim 2, wherein: the indicator comprisesa recommended replacement time (RRT) indicator; the specified pluralityof i2i messages comprise at least N consecutive i2i messages includingthe same RRT indicator; and the IMD sending comprises the IMD sending atleast N consecutive i2i messages including the same RRT indicator to theLP when the IMD has reached its RRT.
 5. The method of claim 2, wherein:the indicator comprises an automatic mode switch (AMS) entry or exitindicator; the specified plurality of i2i messages comprise at least Nconsecutive i2i messages including the same AMS entry or exit indicator;and the IMD sending comprises the IMD sending at least N consecutive i2imessages including the same AMS entry or exit indicator to the LP whenthe IMD determines that a threshold for AMS entry or exit has been met.6. The method of claim 2, wherein: the indicator comprises a magnetentry or exit indicator; the specified plurality of i2i messagescomprise at least N consecutive i2i messages including the same magnetentry or exit indicator; and the IMD sending comprises the IMD sendingat least N consecutive i2i messages including the same magnet entry orexit indicator to the LP when the IMD enters or exits a magnet mode. 7.The method of claim 1, wherein: the specified plurality of i2i messagescomprise at least M out of N i2i messages including the same indicator,where M is a predetermined integer that is equal to or greater than 2,and N is a predetermined integer that is greater than M; and the methodalso comprises the IMD sending at least N i2i messages including thesame indicator to the LP when the IMD makes a specific change to itsoperation that the LP should be informed of, or when the IMD instructsthe LP to make a specific change.
 8. The method of claim 7, wherein: theindicator comprises a pacing rate indicator; the specified plurality ofi2i messages comprise at least M out of N i2i messages including thesame pacing rate indicator; and the IMD sending comprises the IMDsending at least N i2i messages including the same pacing rate indicatorto the LP when the IMD instructs the LP to change a pacing rate.
 9. Themethod of claim 7, wherein: the indicator comprises a recommendedreplacement time (RRT) indicator; the specified plurality of i2imessages comprise at least M out of N i2i messages including the sameRRT indicator; and the IMD sending comprises the IMD sending at least Ni2i messages including the same RRT indicator to the LP when the IMD hasreached its RRT.
 10. The method of claim 7, wherein: the indicatorcomprises an automatic mode switch (AMS) entry or exit indicator; thespecified plurality of i2i messages comprise at least M out of N i2imessages including the same AMS entry or exit indicator; and the IMDsending comprises the IMD sending at least N i2i messages including thesame AMS entry or exit indicator to the LP when the IMD determines thata threshold for AMS entry or exit has been met.
 11. The method of claim7, wherein: the indicator comprises a magnet entry or exit indicator;the specified plurality of i2i messages comprise at least M out of N i2imessages including the same magnet entry or exit indicator; and the IMDsending comprises the IMD sending at least N i2i messages including thesame magnet entry or exit indicator to the LP when the IMD enters orexits a magnet mode.
 12. The method of claim 1, wherein the i2i messagesare transmitted and received via conductive communication; and wherein:the LP comprises a first LP (LP1) implanted in or on a first cardiacchamber and the IMD comprises a second LP (LP2) implanted in or on asecond cardiac chamber; or the IMD comprises a subcutaneous implantablecardioverter defibrillator (S-ICD).
 13. A system comprising: a leadlesspacemaker (LP) configured to be implanted in or on a cardiac chamber ofa patient and configured to pace the cardiac chamber; and an implantablemedical device (IMD) configured to be remotely located relative to theLP; wherein the LP is configured to monitor for implant-to-implant (i2i)messages transmitted by the IMD remotely located relative to the LP;make a change, when the LP receives at least a specified plurality ofi2i messages including a same indicator, based on the same indicatorincluded in the specified plurality of i2i messages received by the LP;and not make the change, when the LP does not receive at least thespecified plurality of the i2i messages including the same indicator.14. The system of claim 13, wherein: the specified plurality of i2imessages comprise at least N consecutive i2i messages including the sameindicator, where N is a predetermined integer that is equal to orgreater than 2; and the IMD is configured to send at least N consecutivei2i messages including the same indicator to the LP when the IMD makes aspecific change to its operation that the LP should be informed of, orwhen the IMD instructs the LP to make a specific change.
 15. The systemof claim 14, wherein: the indicator comprises a pacing rate indicator;the specified plurality of i2i messages comprise at least N consecutivei2i messages including the same pacing rate indicator; and the IMD isconfigured to send at least N consecutive i2i messages including thesame pacing rate indicator to the LP when the IMD instructs the LP tochange a pacing rate.
 16. The system of claim 14, wherein: the indicatorcomprises a recommended replacement time (RRT) indicator; the specifiedplurality of i2i messages comprise at least N consecutive i2i messagesincluding the same RRT indicator; and the IMD is configured to send atleast N consecutive i2i messages including the same RRT indicator to theLP when the IMD has reached its RRT.
 17. The system of claim 14,wherein: the indicator comprises an automatic mode switch (AMS) entry orexit indicator; the specified plurality of i2i messages comprise atleast N consecutive i2i messages including the same AMS entry or exitindicator; and the IMD is configured to send at least N consecutive i2imessages including the same AMS entry or exit indicator to the LP whenthe IMD determines that a threshold for AMS entry or exit has been met.18. The system of claim 14, wherein: the indicator comprises a magnetentry or exit indicator; the specified plurality of i2i messagescomprise at least N consecutive i2i messages including the same magnetentry or exit indicator; and the IMD is configured to send at least Nconsecutive i2i messages including the same magnet entry or exitindicator to the LP when the IMD enters or exits a magnet mode.
 19. Thesystem of claim 13, wherein: the specified plurality of i2i messagescomprise at least M out of N i2i messages including the same indicator,where M is a predetermined integer that is equal to or greater than 2,and N is a predetermined integer that is greater than M; and the IMD isconfigured to send at least N i2i messages including the same indicatorto the LP when the IMD makes a specific change to its operation that theLP should be informed of, or when the IMD instructs the LP to make aspecific change.
 20. The system of claim 19, wherein: the indicatorcomprises a pacing rate indicator; the specified plurality of i2imessages comprise at least M out of N i2i messages including the samepacing rate indicator; and the IMD is configured to send at least N i2imessages including the same pacing rate indicator to the LP when the IMDinstructs the LP to change a pacing rate.
 21. The system of claim 19,wherein: the indicator comprises a recommended replacement time (RRT)indicator; the specified plurality of i2i messages comprise at least Mout of N i2i messages including the same RRT indicator; and the IMD isconfigured to send at least N i2i messages including the same RRTindicator to the LP when the IMD has reached its RRT.
 22. The system ofclaim 19, wherein: the indicator comprises an automatic mode switch(AMS) entry or exit indicator; the specified plurality of i2i messagescomprise at least M out of N i2i messages including the same AMS entryor exit indicator; and the IMD is configured to send at least N i2imessages including the same AMS entry or exit indicator to the LP whenthe IMD determines that a threshold for AMS entry or exit has been met.23. The system of claim 19, wherein: the indicator comprises a magnetentry or exit indicator; the specified plurality of i2i messagescomprise at least M out of N i2i messages including the same magnetentry or exit indicator; and the IMD is configured to send at least Ni2i messages including the same magnet entry or exit indicator to the LPwhen the IMD enters or exits a magnet mode.
 24. The system of claim 13,wherein the i2i messages are transmitted and received via conductivecommunication; and wherein: the LP comprises a first LP (LP1) implantedin or on a first cardiac chamber and the IMD comprises a second LP (LP2)implanted in or on a second cardiac chamber; or the IMD comprises asubcutaneous implantable cardioverter defibrillator (S-ICD).